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v3.22.0.1

Segment Reporting - Narrative (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
HMS |  |  |
Segment Reporting Disclosure [Line Items] |  |  |
Total net revenues (as percentage) | 9.40% | 12.00% | 14.00%

X
- Definition Net revenue from one of the clients as a percentage of total net revenues of the Company for the reporting period. + References No definition available. + Details
Name: | g_PercentageContributionOfSpecificClientInRevenue
Namespace Prefix: | g_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Segment Reporting Disclosure [Line Items] + References No definition available. + Details
Name: | g_SegmentReportingDisclosureLineItems
Namespace Prefix: | g_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=g_HighTechManufacturingAndServicesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |