Reflects the maximum percentage of beneficial ownership that can be acquired as per purchase agreement.

No definition available.

Name:	gpx_BeneficialOwnershipMaximumPercentage
Namespace Prefix:	gpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Reflects the number of common stock shares beneficially owned during the period.

+ References

No definition available.

+ Details

Name:	gpx_CommonStockBeneficialOwnershipNumber
Namespace Prefix:	gpx_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Reflects the percentage of common stock shares beneficially owned during the period.

+ References

No definition available.

+ Details

Name:	gpx_CommonStockBeneficialOwnershipPercentage
Namespace Prefix:	gpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	gpx_CommonStockLineItems
Namespace Prefix:	gpx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of liquidated damages to be paid to holders of shares.

+ References

No definition available.

+ Details

Name:	gpx_LiquidatedDamagesAmount
Namespace Prefix:	gpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the percentage of liquidated damages on purchase price of shares.

+ References

No definition available.

+ Details

Name:	gpx_PurchasePricePercentage
Namespace Prefix:	gpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Reflects the purchase price of securities purchase agreement.

+ References

No definition available.

+ Details

Name:	gpx_SecuritiesPurchaseAgreementPurchasePrice
Namespace Prefix:	gpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the amount of indemnification under the registration statements.

+ References

No definition available.

+ Details

Name:	gpx_ShareholdersIndemnificationAmount
Namespace Prefix:	gpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects shares sold in private placement.

+ References

No definition available.

+ Details

Name:	gpx_SharesSoldInPrivatePlacement
Namespace Prefix:	gpx_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Stock Repurchase Program, Authorized Per Share Range

+ References

No definition available.

+ Details

Name:	gpx_StockRepurchaseProgramAuthorizedPerShareRange
Namespace Prefix:	gpx_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Stock Repurchase Program, Percentage of Outstanding Shares Repurchased

+ References

No definition available.

+ Details

Name:	gpx_StockRepurchaseProgramPercentageofOutstandingSharesRepurchased
Namespace Prefix:	gpx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Costs incurred in connection with the Company's tender offer transaction.

+ References

No definition available.

+ Details

Name:	gpx_Tenderofferexpenses
Namespace Prefix:	gpx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Aggregate number of common shares reserved for future issuance.

+ References

+ Details

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Face amount or stated value per share of common stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

+ Details

Name:	us-gaap_CommonStockSharesOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash outflow to reacquire common and preferred stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsForRepurchaseOfEquity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

+ Details

Name:	us-gaap_StockRepurchasedDuringPeriodShares
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock.

+ References

+ Details

Name:	us-gaap_StockRepurchasedDuringPeriodValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of a stock repurchase plan authorized by an entity's Board of Directors.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramAuthorizedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount remaining of a stock repurchase plan authorized by an entity's Board of Directors.

+ References

No definition available.

+ Details

Name:	us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total cost of shares repurchased divided by the total number of shares repurchased.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

+ Details

Name:	us-gaap_TreasuryStockAcquiredAverageCostPerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=gpx_EquityCompensationAwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gpx_TwoThousandAndElevenPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=gpx_SagardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=gpx_ModifiedDutchAuctionTenderOfferMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareRepurchaseProgramAxis=gpx_StockRepurchaseProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Common Stock (Details Textual) (USD $)	12 Months Ended			0 Months Ended
Common Stock (Details Textual) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 03, 2014	Sep. 29, 2014	Sep. 02, 2014	Dec. 31, 2011	Dec. 31, 2009
Common Stock [Line Items]
Common stock, shares outstanding	17,149,129us-gaap_CommonStockSharesOutstanding			17,086,145us-gaap_CommonStockSharesOutstanding
Repurchases of common stock in the open market	$ 66,640,000	$ 1,747,000	$ 3,433,000
Shares sold in private placement								2,857,143gpx_SharesSoldInPrivatePlacement
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01						$ 0.01
Securities purchase agreement purchase price								20,000,000
Beneficial ownership maximum percentage								23.00%
Purchase price percentage							1.00%gpx_PurchasePricePercentage
Shareholders indemnification amount							25,000gpx_ShareholdersIndemnificationAmount
Sagard [Member]
Common Stock [Line Items]
Share price (in dollars per share)								$ 7.00
Stock beneficially owned	3,514,274
Stock beneficial ownership percentage	20.50%
Maximum [Member]
Common Stock [Line Items]
Liquidated damages amount							2,400,000
Equity Compensation Award [Member]
Common Stock [Line Items]
Shares reserved for future issuance	492,234
2011 Plan [Member]
Common Stock [Line Items]
Shares reserved for future issuance	973,588
Modified Dutch Auction Tender Offer [Member]
Common Stock [Line Items]
Stock repurchase program authorized amount						80,000,000
Stock repurchased during period (shares)				2,127,706
Treasury stock acquired average cost per share					$ 29.00
Payments for repurchase of equity				61,700,000
Tender offer expenses				1,200,000
Percentage of outstanding shares repurchased					11.10%
Modified Dutch Auction Tender Offer [Member] \| Maximum [Member]
Common Stock [Line Items]
Authorized per share range (in dollars per share)						$ 29.00
Modified Dutch Auction Tender Offer [Member] \| Minimum [Member]
Common Stock [Line Items]
Authorized per share range (in dollars per share)						$ 26.00
Stock Repurchase Program [Member]
Common Stock [Line Items]
Stock repurchased during period (shares)	147,000	67,000	180,000
Repurchases of common stock in the open market	3,700,000	1,700,000	3,400,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 600,000