UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No.     )*

GMX RESOURCES INC.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

38011M108

(CUSIP Number)

Marisa Beeney

GSO Capital Partners LP

345 Park Avenue

New York, New York 10154

Tel: (212) 583-5000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

February 27, 2013

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  x

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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This Statement on Schedule 13D (this “Schedule 13D”) relates to the Common Stock, par value $0.001 per share (the “Common Stock”), of GMX Resources Inc., an Oklahoma corporation (the “Issuer”), having its principal executive offices at One Benham Place, 9400 North Broadway, Suite 600, Oklahoma City, Oklahoma 73114.

(a) – (b) This Schedule 13D is being filed by:

The principal business address of each of the GSO Entities and GSO Executives is c/o GSO Capital Partners LP, 345 Park Avenue, New York, NY 10154. The principal business address of each of the Blackstone Entities and Mr. Schwarzman is c/o The Blackstone Group, 345 Park Avenue, New York, NY 10154.

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The principal business of the GSO Funds is investing in both public and private non-investment grade and non-rated securities, including leveraged loans, high yield bonds, distressed securities, second lien loans, mezzanine securities, equity securities, credit derivatives and other investments.

The principal business of GSO Palmetto Opportunistic Associates LLC is performing the functions of, and serving as, the general partner of GSO Palmetto Opportunistic Investment Partners LP. The principal business of GSO Credit-A Associates LLC is performing the functions of, and serving as, the general partner of GSO Credit-A Partners LP. The principal business of GSO Holdings I L.L.C. is performing the functions of, and serving as, the managing member (or similar position) of and member or equity holder in each of GSO Palmetto Opportunistic Associates LLC and GSO Credit-A Associates LLC.

GSO Special Situations Overseas Master Fund Ltd. is a wholly owned subsidiary of GSO Special Situations Overseas Fund Ltd. GSO Capital Partners LP serves as the investment manager of GSO Special Situations Fund LP, GSO Special Situations Overseas Master Fund Ltd. and GSO Special Situations Overseas Fund Ltd.

The principal business of GSO Advisor Holdings L.L.C. is performing the functions of, and serving as, the general partner of GSO Capital Partners LP. The principal business of Blackstone Holdings I L.P. is performing the functions of, and serving as, the managing member (or similar position) of and member or equity holder in GSO Advisor Holdings L.L.C., GSO Advisor Holdings L.L.C. and other affiliated entities. The principal business of Blackstone Holdings I/II GP Inc. is performing the functions of, and serving as, the general partner (or similar position) of Blackstone Holdings I L.P. and other affiliated Blackstone entities. The principal business of The Blackstone Group L.P. is performing the functions of, and serving as, the controlling shareholder of Blackstone Holdings I/II GP Inc. and other affiliated Blackstone entities. The principal business of Blackstone Group Management L.L.C. is performing the functions of, and serving as, the general partner of The Blackstone Group L.P.

The principal occupation of Mr. Stephen A. Schwarzman is serving as an executive of Blackstone Group Management L.L.C. The principal occupation of each of Messrs. Goodman, Smith and Ostrover is serving as an executive of GSO Capital Partners LP.

(c) During the last five years, none of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(d) During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and, as a result of such proceeding, was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(e) See Item 2(a)-(b) above for citizenship of each of the Reporting Persons.

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The aggregate purchase price of the 519,805 shares of Common Stock purchased by the GSO Funds on December 7, 2012 of this Schedule 13D was approximately $1,248,609.

The GSO Funds’ payment of the aggregate purchase price was funded by cash on hand.

The shares of Common Stock held by the GSO Funds were acquired directly from the Company in a private placement transaction on December 7, 2012. The GSO Funds currently hold such shares for investment purposes, subject to the following.

In connection with the Company’s previous announcement that it is exploring financing alternatives, including a potential restructuring of the Company’s balance sheet in light of its current liquidity and cash needs, certain of the Reporting Persons have engaged in preliminary discussions with the Company and its representatives regarding potential transactions that may be deemed to have the purpose or effect of changing or influencing control of the Issuer, or that may be in connection with or relating to such Reporting Persons as a participant in any transaction having that purpose or effect, including activities which may relate to items described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

The Reporting Persons intend to review on a continuing basis the GSO Funds’ investment in the Issuer. As a result of the Reporting Persons’ continuous review and evaluation of the business of the Issuer, the Reporting Persons may communicate with the board of directors of the Issuer, members of management and/or other security holders from time to time with respect to operational, strategic, financial or governance matters or otherwise work with management and the board of directors with a view to maximizing security holder value. Subject to the agreements described herein, the Reporting Persons may seek to sell or otherwise dispose of some or all of the Issuer’s securities (which may include, but is not limited to, transferring some or all of such securities to its affiliates or distributing some or all of such securities to such Reporting Person’s respective partners, members or beneficiaries, as applicable) from time to time, and/or may seek to acquire additional securities of the Issuer (which may include rights or securities exercisable or convertible into securities of the Issuer) from time to time, in each case, in open market or private transactions, block sales or otherwise. Any transaction that the Reporting Persons may pursue may be made at any time and from time to time without prior notice and will depend on a variety of factors, including, without limitation, the price and availability of the Issuer’s securities, subsequent developments affecting the Issuer, the Issuer’s business and the Issuer’s prospects, other investment and business opportunities available to the Reporting Persons, general industry and economic conditions, the securities markets in general, tax considerations and other factors deemed relevant by the Reporting Persons.

Except as described in this Item 4 of Schedule 13D, the Reporting Persons do not have any present plans or proposals that relate to or would result in any of the actions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D, the Reporting Persons, at any time and from time to time, may review, reconsider and change their position and/or change their purpose and/or develop such plans and may seek to influence management or the board of directors of the

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Issuer with respect to the business and affairs of the Issuer, and may from time to time consider pursuing or proposing any such transactions and, in connection therewith, may discuss, evaluate and/or pursue any such transactions with advisors, the Issuer or other persons.

(a) – (b) Based on information provided by the Issuer, the following disclosure assumes that there were 7,405,783 shares of Common Stock of the Issuer outstanding as of February 13, 2013.

Based on this number of outstanding shares of Common Stock, each of the Reporting Persons is or may be deemed to be the beneficial owner of the percentage of the total number of outstanding shares of Common Stock listed on such Reporting Person’s cover page.

As of the date hereof, GSO Palmetto Opportunistic Investment Partners LP and GSO Credit-A Partners LP respectively hold 36,354 and 35,233 shares of Common Stock of the Issuer reported on this Schedule 13D and GSO Special Situations Fund LP and GSO Special Situations Overseas Master Fund Ltd. respectively hold 233,145 and 215,073 shares of Common Stock of the Issuer reported on this Schedule 13D.

As of the date hereof GSO Palmetto Opportunistic Investment Partners LP holds $6,810,140 aggregate principal amount of Senior Secured Notes due December 2017 of the Issuer (the “2017 Notes”); GSO Credit-A Partners LP holds $6,600,560 aggregate principal amount of 2017 Notes; GSO Special Situations Fund LP holds $44,735,682 aggregate principal amount of 2017 Notes and $2,491,000 aggregate principal amount of 4.50% Senior Convertible Notes due 2015 of the Issuer (the “2015 Notes”); GSO Special Situations Overseas Master Fund Ltd. holds $41,270,606 aggregate principal amount of 2017 Notes and $2,305,000 aggregate principal amount of 2015 Notes; and Blackstone / GSO MAK Fund LP, an affiliate of the GSO Funds, holds $5,100,000 aggregate principal amount of 2017 Notes. The 2015 Notes are not currently convertible and therefor the Common Stock underlying the 2015 Notes is not reported on this Schedule 13D.

GSO Palmetto Opportunistic Associates LLC is the general partner of GSO Palmetto Opportunistic Investment Partners LP. GSO Credit-A Associates LLC is the general partner of GSO Credit-A Partners LP. GSO Holdings I L.L.C. is the managing member of each of GSO Palmetto Opportunistic Associates LLC and GSO Credit-A Associates LLC.

GSO Special Situations Overseas Master Fund Ltd is a wholly owned subsidiary of GSO Special Situations Overseas Fund Ltd. GSO Capital Partners LP serves as the investment manager of GSO Special Situations Fund LP, GSO Special Situations Overseas Master Fund Ltd. and GSO Special Situations Overseas Fund Ltd.

GSO Advisor Holdings L.L.C. is the general partner of GSO Capital Partners LP. Blackstone Holdings I L.P. is the managing member of each of GSO Holdings I L.L.C. and GSO Advisor Holdings L.L.C. Blackstone Holdings I/II GP Inc. is the general partner of Blackstone Holdings I L.P. The Blackstone Group L.P. is the controlling shareholder of Blackstone Holdings I/II GP Inc. Blackstone Group Management L.L.C. is the general partner of The Blackstone Group L.P. Blackstone Group Management L.L.C. is wholly owned by Blackstone’s senior managing directors and controlled by its founder, Stephen A. Schwarzman.

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In addition, each of Bennett J. Goodman, J. Albert Smith III and Douglas I. Ostrover may be deemed to have shared voting power and/or investment power with respect to the Common Stock held by the GSO Funds.

Each such Reporting Person may be deemed to beneficially own the Common Shares beneficially owned by the GSO Funds directly or indirectly controlled by it or him, but neither the filing of this Schedule 13G nor any of its contents shall be deemed to constitute an admission that any Reporting Person (other than the GSO Funds identified is directly holding shares of Common Stock) is the beneficial owner of Common Stock referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose and each of the Reporting Persons expressly disclaims beneficial ownership of such shares of Common Stock and any assertion or presumption that it and the other persons on whose behalf this statement is filed constitute a “group.”

GSO Capital Partners LP is the investment manager of each of GSO Special Situations Fund LP, GSO Special Situations Overseas Master Fund, Ltd., GSO Special Situations Overseas Benefit Plan Fund, Ltd. and GSO Capital Opportunities Fund LP, and in that respect holds discretionary investment authority for each of them. GSO Advisor Holdings L.L.C. is the general partner of GSO Capital Partners LP, and in that capacity directs its operations. Blackstone Holdings I L.P. is the sole member of GSO Advisor Holdings L.L.C., and in that capacity, directs its operations. Blackstone Holdings I/II GP Inc. is the general partner of Blackstone Holdings I L.P., and in that capacity, directs its operations. The Blackstone Group L.P. is the controlling shareholder of Blackstone Holdings I/II GP Inc., and in that capacity, directs its operations. Blackstone Group Management L.L.C. is the general partner of The Blackstone Group L.P., and in that capacity directs its operations. Blackstone Group Management L.L.C. is wholly owned by its senior managing directors and controlled by its founder, Stephen A. Schwarzman. In addition, each of Bennett J. Goodman, J. Albert Smith III and Douglas I. Ostrover may have shared voting and dispositive power with respect to the shares of Common Stock.

The aggregate number and percentage of the shares of Common Stock beneficially owned by each Reporting Person and, for each Reporting Person, the number of shares as to which there is sole power to vote or to direct the vote, shared power to vote or to direct the vote, sole power to dispose or to direct the disposition, or shared power to dispose or to direct the disposition are set forth on rows 7 through 11 and row 13 of the cover pages of this Schedule 13D.

Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that any of the Reporting Persons (other than the GSO Funds, to the extent they directly hold the shares of Common Stock reported on this Schedule 13D) is the beneficial owner of the Common Stock referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose, and such beneficial ownership is expressly disclaimed.

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(c) Except as set forth herein, none of the Reporting Persons has engaged in any transaction during the past 60 days in any shares of Common Stock.

(d) Not applicable.

(e) Not applicable.

Other than as disclosed in Item 5 above, none of the Reporting Persons has any contract, arrangement, understanding or relationship (legal or otherwise) with any person with respect to any securities of the Issuer, including but not limited to, any contract, arrangement, understanding or relationship concerning the transfer or voting of any securities of the Issuer, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guaranties of profits, division of profits or loss or the giving or withholding of proxies.

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: March 11, 2013

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Exhibit A

JOINT FILING AGREEMENT

Pursuant to and in accordance with the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder (the “Exchange Act”) the undersigned hereby agree to the joint filing of GSO Palmetto Opportunistic Investment Partners LP, GSO Credit-A Partners LP, GSO Special Situations Fund LP, GSO Special Situations Overseas Master Fund Ltd, GSO Special Situations Overseas Fund Ltd., GSO Holdings I L.L.C., GSO Capital Partners LP, GSO Advisor Holdings L.L.C., Blackstone Holdings I L.P., Blackstone Holdings I/II GP Inc., The Blackstone Group L.P., Blackstone Group Management L.L.C., Bennett J. Goodman, J. Albert Smith III, Douglas I. Ostrover, and Stephen A. Schwarzman, on behalf of each of them of any filing required by such party under Section 13 or Section 16 of the Exchange Act or any rule or regulation thereunder (including any amendment, restatement, supplement, and/or exhibit thereto) with respect to securities of GMX Resources Inc., a Delaware corporation, and further agree to the filing, furnishing, and/or incorporation by reference of this Agreement as an exhibit thereto. This Agreement shall remain in full force and effect until revoked by any party hereto in a signed writing provided to each other party hereto, and then only with respect to such revoking party. This Agreement may be executed in any number of counterparts all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned hereby execute this Agreement this 11th day of March 2013.

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Exhibit B

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that I, BENNETT J. GOODMAN, hereby make, constitute and appoint MARISA BEENEY, acting individually, as my agent and attorney-in-fact for the purpose of executing in my name, (a) in my personal capacity or (b) in my capacity as an executive, member of or in other capacities with GSO Capital Partners LP and each of its affiliates or entities advised by me or GSO Capital Partners LP, all documents, certificates, instruments, statements, filings and agreements (“documents”) to be filed with or delivered to any foreign or domestic governmental or regulatory body or required or requested by any other person or entity pursuant to any legal or regulatory requirement relating to the acquisition, ownership, management or disposition of securities, futures contracts or other investments, and any other documents relating or ancillary thereto, including without limitation all documents relating to filings with any stock exchange, self-regulatory association, the Commodities Futures Trading Commission and National Futures Association, the United States Securities and Exchange Commission (the “SEC”) pursuant to the Securities Act of 1933 or the Securities Exchange Act of 1934 (the “Act”) and the rules and regulations promulgated thereunder, including all documents relating to the beneficial ownership of securities required to be filed with the SEC pursuant to Section 13(d) or Section 16(a) of the Act and any information statements on Form 13F required to be filed with the SEC pursuant to Section 13(f) of the Act, including without limitation Form 3, 4, 5, 144, or Schedules 13D, 13F and 13G and any amendments to said forms or schedules, in each case, as determined by such person to be necessary or appropriate. Any such determination shall be conclusively evidenced by such person’s execution, delivery, furnishing and/or filing of the applicable document.

All past acts of the attorneys-in-fact in furtherance of the foregoing are hereby ratified and confirmed.

This power of attorney shall be valid from the date hereof until the date revoked in writing by the undersigned, and this power of attorney does not revoke or replace any other power of attorney that the undersigned has previously granted.

IN WITNESS WHEREOF, I have executed this instrument as of the 8th day of June, 2010.

Exhibit C

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that I, J. ALBERT SMITH III, hereby make, constitute and appoint MARISA BEENEY, acting individually, as my agent and attorney-in-fact for the purpose of executing in my name, (a) in my personal capacity or (b) in my capacity as an executive, member of or in other capacities with GSO Capital Partners LP and each of its affiliates or entities advised by me or GSO Capital Partners LP, all documents, certificates, instruments, statements, filings and agreements (“documents”) to be filed with or delivered to any foreign or domestic governmental or regulatory body or required or requested by any other person or entity pursuant to any legal or regulatory requirement relating to the acquisition, ownership, management or disposition of securities, futures contracts or other investments, and any other documents relating or ancillary thereto, including without limitation all documents relating to filings with any stock exchange, self-regulatory association, the Commodities Futures Trading Commission and National Futures Association, the United States Securities and Exchange Commission (the “SEC”) pursuant to the Securities Act of 1933 or the Securities Exchange Act of 1934 (the “Act”) and the rules and regulations promulgated thereunder, including all documents relating to the beneficial ownership of securities required to be filed with the SEC pursuant to Section 13(d) or Section 16(a) of the Act and any information statements on Form 13F required to be filed with the SEC pursuant to Section 13(f) of the Act, including without limitation Form 3, 4, 5, 144, or Schedules 13D, 13F and 13G and any amendments to said forms or schedules, in each case, as determined by such person to be necessary or appropriate. Any such determination shall be conclusively evidenced by such person’s execution, delivery, furnishing and/or filing of the applicable document.

All past acts of the attorneys-in-fact in furtherance of the foregoing are hereby ratified and confirmed.

This power of attorney shall be valid from the date hereof until the date revoked in writing by the undersigned, and this power of attorney does not revoke or replace any other power of attorney that the undersigned has previously granted.

IN WITNESS WHEREOF, I have executed this instrument as of the 8th day of June, 2010.

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Exhibit D

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that I, DOUGLAS I. OSTROVER, hereby make, constitute and appoint MARISA BEENEY, acting individually, as my agent and attorney-in-fact for the purpose of executing in my name, (a) in my personal capacity or (b) in my capacity as an executive, member of or in other capacities with GSO Capital Partners LP and each of its affiliates or entities advised by me or GSO Capital Partners LP, all documents, certificates, instruments, statements, filings and agreements (“documents”) to be filed with or delivered to any foreign or domestic governmental or regulatory body or required or requested by any other person or entity pursuant to any legal or regulatory requirement relating to the acquisition, ownership, management or disposition of securities, futures contracts or other investments, and any other documents relating or ancillary thereto, including without limitation all documents relating to filings with any stock exchange, self-regulatory association, the Commodities Futures Trading Commission and National Futures Association, the United States Securities and Exchange Commission (the “SEC”) pursuant to the Securities Act of 1933 or the Securities Exchange Act of 1934 (the “Act”) and the rules and regulations promulgated thereunder, including all documents relating to the beneficial ownership of securities required to be filed with the SEC pursuant to Section 13(d) or Section 16(a) of the Act and any information statements on Form 13F required to be filed with the SEC pursuant to Section 13(f) of the Act, including without limitation Form 3, 4, 5, 144, or Schedules 13D, 13F and 13G and any amendments to said forms or schedules, in each case, as determined by such person to be necessary or appropriate. Any such determination shall be conclusively evidenced by such person’s execution, delivery, furnishing and/or filing of the applicable document.

All past acts of the attorneys-in-fact in furtherance of the foregoing are hereby ratified and confirmed.

This power of attorney shall be valid from the date hereof until the date revoked in writing by the undersigned, and this power of attorney does not revoke or replace any other power of attorney that the undersigned has previously granted.

IN WITNESS WHEREOF, I have executed this instrument as of the 8th day of June, 2010.

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