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Income taxes - Schedule of change in total valuation allowance for deferred tax assets (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Income Tax Disclosure [Abstract] |  |  |
Opening valuation allowance | $ 206,011 | $ 62,628 | $ 51,986
Reduction during the year | (1,206) | (35,662) | (4,240)
Addition during the year | 7,387 | 179,045 | 14,882
Closing valuation allowance | $ 212,192 | $ 206,011 | $ 62,628

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- Definition The amount of the increase in the period in the valuation allowance for deferred tax assets. + References No definition available. + Details
Name: | g_ValuationAllowanceDeferredTaxAssetChangeInAmountAdditions
Namespace Prefix: | g_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The amount of the reduction in the period in the valuation allowance for deferred tax assets. + References No definition available. + Details
Name: | g_ValuationAllowanceDeferredTaxAssetChangeInAmountReductions
Namespace Prefix: | g_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=121826272&loc=d3e32537-109319 + Details
Name: | us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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