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Net Revenues - Narrative (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Revenues [Abstract] |  |  |  |
Billing cycle period |  |  | 30 days |
Revenue recognized | $ 60,756 | $ 51,201 | $ 128,628 | $ 83,189

X
- 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

X
- Definition Amount of revenue recognized that was previously reported as deferred or unearned revenue. + References No definition available. + Details
Name: | us-gaap_DeferredRevenueRevenueRecognized1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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