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Income taxes - Schedule of change in total valuation allowance for deferred tax assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	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

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Namespace Prefix:	g_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- 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

X

- 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

X

- References

No definition available.

+ Details

Name:	us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration