Expectation of the average long term rate of return expected in years.

No definition available.

Name:	g_AverageLongTermRateOfReturnExpectedPeriod
Namespace Prefix:	g_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred compensation arrangement, employer discretionary, vesting percentage.

+ References

No definition available.

+ Details

Name:	g_DeferredCompensationArrangementEmployerDiscretionaryVestingPercentage
Namespace Prefix:	g_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Deferred compensation arrangement, employer discretionary, vesting period

+ References

No definition available.

+ Details

Name:	g_DeferredCompensationArrangementEmployerDiscretionaryVestingPeriod
Namespace Prefix:	g_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred compensation arrangement with individual, participants’ deferrals vesting percentage.

+ References

No definition available.

+ Details

Name:	g_DeferredCompensationArrangementWithIndividualParticipantsDeferralsVestingPercentage
Namespace Prefix:	g_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred compensation arrangement with individual, qualifying bonus compensation percentage.

+ References

No definition available.

+ Details

Name:	g_DeferredCompensationArrangementWithIndividualQualifyingBonusCompensationPercentage
Namespace Prefix:	g_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Benefit Plan Under Funded Status Of Plan

+ References

No definition available.

+ Details

Name:	g_DefinedBenefitPlanUnderFundedStatusOfPlan
Namespace Prefix:	g_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Income (expense) on deferred compensation arrangement liability.

+ References

No definition available.

+ Details

Name:	g_IncomeExpenseOnDeferredCompensationArrangementLiability
Namespace Prefix:	g_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income (expense) on deferred compensation arrangement, plan assets.

+ References

No definition available.

+ Details

Name:	g_IncomeExpenseOnDeferredCompensationArrangementPlanAssets
Namespace Prefix:	g_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(17))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Commitment made to pay deferred cash remuneration expressed as a percentage of the individual's base salary.

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualCashAwardsGrantedPercentage
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Maximum term of the deferred compensation arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 55
-Paragraph 7
-URI https://asc.fasb.org/extlink&oid=6409961&loc=d3e20487-108367

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualMaximumContractualTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum period the individual is required to perform services to be fully vested under the deferred compensation arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 55
-Paragraph 7
-URI https://asc.fasb.org/extlink&oid=6409961&loc=d3e20487-108367

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer).

+ References

No definition available.

+ Details

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

- Definition

Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (g)
-URI https://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 18
-URI https://asc.fasb.org/extlink&oid=123450688&loc=d3e4587-114921

+ Details

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

- Details

Name:	us-gaap_RetirementPlanSponsorLocationAxis=us-gaap_ForeignPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=g_VestingPeriodOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=g_VestingPeriodTwoMember
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_RetirementPlanNameAxis=g_BenefitObligationsOfPhilippinesPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Benefit Plans - Narrative (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jul. 01, 2018	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Net projected benefit obligation and plan assets for underfunded (including unfunded) defined benefit obligation plans		$ 12,386	$ 13,739
Expected contributions in fiscal 2022		7,860
Deferred compensation plan liability		39,654	38,007
Cash surrender value of policies		40,261	38,584
Change in fair value of plan assets		(7,580)	4,229	$ 4,164
Change in fair value of deferred compensation liabilities		$ (7,610)	$ 4,094	$ 4,120
Foreign Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Participant deferrals vesting ( in percentage)	100.00%
Employer discretionary vesting period	2 years
Earnings receivable minimum term	2 years
Earnings receivable lump sum or annual installment maximum terms	15 years
One-year anniversary of approval of contribution \| Foreign Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employer discretionary vesting (in percentage)	50.00%
Two-year anniversary of approval of contribution \| Foreign Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Employer discretionary vesting (in percentage)	50.00%
Minimum \| Foreign Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Individual qualifying base compensation (in percentage)	1.00%
Individual qualifying bonus compensation (in percentage)	1.00%
Maximum \| Foreign Plan
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Individual qualifying base compensation (in percentage)	80.00%
Individual qualifying bonus compensation (in percentage)	100.00%
Benefit obligations of Philippines Plan \| Minimum
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Expectation of the average long term rate of return expected, years		15 years
Benefit obligations of Philippines Plan \| Maximum
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Expectation of the average long term rate of return expected, years		20 years