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Other income (expense), net - Schedule of other income (expense), net (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
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Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Other Nonoperating Income (Expense) [Abstract] |  |  |  |
Other income (expense) | $ 1,480 | $ 960 | $ 8,966 | $ 946
Other income (expense), net | $ 1,480 | $ 960 | $ 8,966 | $ 946

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- Definition Other sundry nonoperating income expense. + References No definition available. + Details
Name: | g_OtherSundryNonoperatingIncomeExpense
Namespace Prefix: | g_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of income (expense) related to nonoperating activities, classified as other. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.9) -URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227 + Details
Name: | us-gaap_OtherNonoperatingIncomeExpense
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_OtherNonoperatingIncomeExpenseAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration