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Net revenues - Narrative (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Revenues [Abstract] |  |
Billing cycle period | 30 days |
Contract liabilities balance | $ 141,774 | $ 102,893

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- Definition Billing cycle period. + References No definition available. + Details
Name: | g_BillingCyclePeriod
Namespace Prefix: | g_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045 + Details
Name: | us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_RevenuesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration