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%

Net revenue from one of the clients as a percentage of total net revenues of the Company for the reporting period.

No definition available.

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