FAIR VALUE MEASUREMENTS - Financial Instruments Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024
Fair Value on a Recurring Basis
Marketable securities	$ 352,990	$ 422,069
Recurring basis
Fair Value on a Recurring Basis
Assets	393,641	474,121
Recurring basis \| U.S. Treasury securities
Fair Value on a Recurring Basis
Marketable securities	22,785	30,570
Recurring basis \| Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Marketable securities	4,001	8,748
Recurring basis \| Commercial paper
Fair Value on a Recurring Basis
Marketable securities	123,995	185,201
Recurring basis \| Corporate notes
Fair Value on a Recurring Basis
Marketable securities	204,695	202,527
Recurring basis \| Money market funds
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	37,892	46,802
Recurring basis \| Certificate of deposit
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	273	273
Recurring basis \| Level 1
Fair Value on a Recurring Basis
Assets	38,165	47,075
Recurring basis \| Level 1 \| U.S. Treasury securities
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 1 \| Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 1 \| Commercial paper
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 1 \| Corporate notes
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 1 \| Money market funds
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	37,892	46,802
Recurring basis \| Level 1 \| Certificate of deposit
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	273	273
Recurring basis \| Level 2
Fair Value on a Recurring Basis
Assets	355,476	427,046
Recurring basis \| Level 2 \| U.S. Treasury securities
Fair Value on a Recurring Basis
Marketable securities	22,785	30,570
Recurring basis \| Level 2 \| Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Marketable securities	4,001	8,748
Recurring basis \| Level 2 \| Commercial paper
Fair Value on a Recurring Basis
Marketable securities	123,995	185,201
Recurring basis \| Level 2 \| Corporate notes
Fair Value on a Recurring Basis
Marketable securities	204,695	202,527
Recurring basis \| Level 2 \| Money market funds
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	0	0
Recurring basis \| Level 2 \| Certificate of deposit
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	0	0
Recurring basis \| Level 3
Fair Value on a Recurring Basis
Assets	0	0
Recurring basis \| Level 3 \| U.S. Treasury securities
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 3 \| Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 3 \| Commercial paper
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 3 \| Corporate notes
Fair Value on a Recurring Basis
Marketable securities	0	0
Recurring basis \| Level 3 \| Money market funds
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	0	0
Recurring basis \| Level 3 \| Certificate of deposit
Fair Value on a Recurring Basis
Cash, cash equivalents and restricted cash	$ 0	$ 0

Cash And Cash Equivalents And Restricted Cash, Fair Value Disclosure

No definition available.

Name:	gern_CashAndCashEquivalentsAndRestrictedCashFairValueDisclosure
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of asset recognized for present right to economic benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryNotesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateNoteSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_MoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CashAndCashEquivalentsAxis=us-gaap_CertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: