Debt Instrument, Covenant, Leverage Ratio

No definition available.

Name:	gpx_DebtInstrumentCovenantLeverageRatio
Namespace Prefix:	gpx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Debt Instrument, Covenant, Minimum Interest Expense Coverage

+ References

No definition available.

+ Details

Name:	gpx_DebtInstrumentCovenantMinimumInterestExpenseCoverage
Namespace Prefix:	gpx_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28555-108399

+ Details

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

- Definition

The effective interest rate during the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateDuringPeriod
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=gpx_FinancingSecurityAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=gpx_OvernightBankFundingRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gpx_DebtCovenantPeriodAxis=gpx_FiscalQuartersThroughMarch312020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	gpx_DebtCovenantPeriodAxis=gpx_FiscalQuartersThroughDecember312020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_ForeignLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=gpx_AccordionFeatureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=gpx_SwingLineLoanCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (Details)		3 Months Ended
Debt (Details)	Nov. 30, 2018 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020	Dec. 31, 2020 USD ($)	May 07, 2020 USD ($)
Short-term Debt [Line Items]
Long-term debt		$ 0		$ 12,748,000
Debt Issuance Costs, Net		1,000,000.0
Financing Security Agreement [Member] \| Base Rate [Member] \| Minimum
Short-term Debt [Line Items]
Interest rate spread	0.25%
Financing Security Agreement [Member] \| Base Rate [Member] \| Maximum
Short-term Debt [Line Items]
Interest rate spread	1.25%
Financing Security Agreement [Member] \| LIBOR
Short-term Debt [Line Items]
Interest rate spread	1.00%
Financing Security Agreement [Member] \| LIBOR \| Minimum
Short-term Debt [Line Items]
Interest rate spread	1.25%
Financing Security Agreement [Member] \| LIBOR \| Maximum
Short-term Debt [Line Items]
Interest rate spread	2.25%
Financing Security Agreement [Member] \| Overnight Bank Funding Rate [Member]
Short-term Debt [Line Items]
Interest rate spread	0.50%
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Available borrowings		$ 87,100,000
Weighted average interest rate		1.90%	3.90%
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Line of Credit [Member]
Short-term Debt [Line Items]
Borrowing capacity	$ 200,000,000				$ 140,000,000
Leverage ratio		0.0
Minimum interest expense coverage		14.4			3.0
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Line of Credit [Member] \| Fiscal Quarters Through March 31, 2020 [Member]
Short-term Debt [Line Items]
Leverage ratio					3.00
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Line of Credit [Member] \| Minimum \| Fiscal Quarters Through December 31, 2020 [Member]
Short-term Debt [Line Items]
Leverage ratio					3.0
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Line of Credit [Member] \| Maximum \| Fiscal Quarters Through December 31, 2020 [Member]
Short-term Debt [Line Items]
Leverage ratio					3.75
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Foreign Line of Credit [Member]
Short-term Debt [Line Items]
Borrowing capacity	20,000,000
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Accordion Feature [Member]
Short-term Debt [Line Items]
Borrowing capacity	100,000,000
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Letter of Credit [Member]
Short-term Debt [Line Items]
Borrowing capacity	20,000,000
Revolving Credit Facility [Member] \| Financing Security Agreement [Member] \| Swing Line Loan Credit [Member]
Short-term Debt [Line Items]
Borrowing capacity	$ 20,000,000