LOANS AND SERVICING - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Allowance for loan losses
Beginning Balance	$ 2,605	$ 2,408
Charge-offs	(3)
Recoveries	1	1
(Benefit) provision	39	74
Ending Balance	2,642	2,483
Ending balance: Individually evaluated for impairment	7		$ 7
Ending balance: Collectively evaluated for impairment	2,635		2,598
Loans
Ending Balance: Individually evaluated for impairment	2,188		2,052
Ending Balance: Collectively evaluated for impairment	279,481		277,440
Total Loans	281,669		279,492
Residential loans:
Loans
Total Loans	108,337		106,069
One- to four- family, residential loans
Allowance for loan losses
Beginning Balance	171	197
(Benefit) provision	5	(17)
Ending Balance	176	180
Ending balance: Collectively evaluated for impairment	176		171
Loans
Ending Balance: Individually evaluated for impairment	1,529		1,543
Ending Balance: Collectively evaluated for impairment	91,340		88,647
Total Loans	92,869		90,190
Home equity loans and lines of credit
Allowance for loan losses
Beginning Balance	238	276
Recoveries	1	1
(Benefit) provision	(7)	(17)
Ending Balance	232	260
Ending balance: Collectively evaluated for impairment	232		238
Loans
Ending Balance: Individually evaluated for impairment	296		143
Ending Balance: Collectively evaluated for impairment	15,172		15,736
Total Loans	15,468		15,879
Commercial loans:
Loans
Total Loans	151,855		148,087
One-to-four family investment property
Allowance for loan losses
Beginning Balance	79	90
(Benefit) provision	7	(7)
Ending Balance	86	83
Ending balance: Individually evaluated for impairment	7		7
Ending balance: Collectively evaluated for impairment	79		72
Loans
Ending Balance: Individually evaluated for impairment	85		85
Ending Balance: Collectively evaluated for impairment	14,315		12,996
Total Loans	14,400		13,081
Multi-family real estate
Allowance for loan losses
Beginning Balance	234	233
(Benefit) provision	(4)	(7)
Ending Balance	230	226
Ending balance: Collectively evaluated for impairment	230		234
Loans
Ending Balance: Collectively evaluated for impairment	30,214		30,748
Total Loans	30,214		30,748
Commercial real estate
Allowance for loan losses
Beginning Balance	1,262	1,021
(Benefit) provision	(3)	105
Ending Balance	1,259	1,126
Ending balance: Collectively evaluated for impairment	1,259		1,262
Loans
Ending Balance: Individually evaluated for impairment	278		281
Ending Balance: Collectively evaluated for impairment	83,112		83,302
Total Loans	83,390		83,583
Commercial business loan
Allowance for loan losses
Beginning Balance	364	305
(Benefit) provision	55	33
Ending Balance	419	338
Ending balance: Collectively evaluated for impairment	419		364
Loans
Ending Balance: Collectively evaluated for impairment	23,851		20,675
Total Loans	23,851		20,675
Construction loans
Loans
Total Loans	21,327		25,154
One-to-four family, construction loans
Allowance for loan losses
Beginning Balance	104	113
(Benefit) provision	(22)	26
Ending Balance	82	139
Ending balance: Collectively evaluated for impairment	82		104
Loans
Ending Balance: Collectively evaluated for impairment	9,368		12,599
Total Loans	9,368		12,599
Multi-family
Allowance for loan losses
Beginning Balance	47	12
(Benefit) provision	(10)	2
Ending Balance	37	14
Ending balance: Collectively evaluated for impairment	37		47
Loans
Ending Balance: Collectively evaluated for impairment	4,505		5,725
Total Loans	4,505		5,725
Non-residential
Allowance for loan losses
Beginning Balance	104	91
(Benefit) provision	10	6
Ending Balance	114	97
Ending balance: Collectively evaluated for impairment	114		104
Loans
Ending Balance: Collectively evaluated for impairment	7,454		6,830
Total Loans	7,454		6,830
Consumer
Allowance for loan losses
Beginning Balance	2	2
Charge-offs	(3)
(Benefit) provision	2
Ending Balance	1	2
Ending balance: Collectively evaluated for impairment	1		2
Loans
Ending Balance: Collectively evaluated for impairment	150		182
Total Loans	150		$ 182
Unallocated
Allowance for loan losses
Beginning Balance		68
(Benefit) provision	6	(50)
Ending Balance	6	$ 18
Ending balance: Collectively evaluated for impairment	$ 6

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossAdditionalInformationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_NotesReceivableGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953423-111524

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_OneToFourFamilyResidentialMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_OneToFourFamilyInvestmentPropertyCommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_MultiFamilyRealEstateCommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_CommercialBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_OneToFourFamilyConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_MultiFamilyConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_NonResidentialConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=gtwn_UnallocatedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: