Accrued end of term charge.

No definition available.

Name:	gern_AccruedEndOfTermCharge
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

First date to available to drawdown any amount under a tranche.

+ References

No definition available.

+ Details

Name:	gern_BorrowingAvailableStartDate
Namespace Prefix:	gern_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Covenant Minimum Cash Balance

+ References

No definition available.

+ Details

Name:	gern_DebtCovenantMinimumCashBalance
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of loan amount outstanding to be held as minimum cash balance under debt covenant.

+ References

No definition available.

+ Details

Name:	gern_DebtCovenantMinimumCashBalancePercentageOfLoanAmount
Namespace Prefix:	gern_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum percentage of six-month product revenue forecast to satisfy minimum cash balance debt covenant.

+ References

No definition available.

+ Details

Name:	gern_DebtCovenantPercentageOfProductRevenueAgainstForecast
Namespace Prefix:	gern_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Prepayment charge assessed upon debt prepayment occurring 36 months after June 30, 2022 under second amendment.

+ References

No definition available.

+ Details

Name:	gern_DollarPrepaymentCharge36MonthsAfterEffectiveDate
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Last date to drawdown any amounts under a tranche.

+ References

No definition available.

+ Details

Name:	gern_ExpiryDateOfTranche
Namespace Prefix:	gern_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum market capitalization required for this option to be available to meet the minimum cash balance debt covenant.

+ References

No definition available.

+ Details

Name:	gern_MarketCapMinimumToUseOptionTwo
Namespace Prefix:	gern_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Minimum percentage of net product revenues maintenance period.

+ References

No definition available.

+ Details

Name:	gern_MinimumPercentageOfNetProductRevenuesMaintenancePeriod
Namespace Prefix:	gern_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of prepayment amount to be paid upon prepayment under debt instrument.

+ References

No definition available.

+ Details

Name:	gern_PercentageOfPrepaymentCharge
Namespace Prefix:	gern_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

No definition available.

+ Details

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

- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(3))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823

+ Details

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

- Definition

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI https://asc.fasb.org/extlink&oid=124435984&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

+ Details

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

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

+ Details

Name:	us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=124435984&loc=d3e28555-108399

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, excluding unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI https://asc.fasb.org/extlink&oid=123466204&loc=SL6031897-161870

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69C
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495737-112612

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 55
-Paragraph 69B
-URI https://asc.fasb.org/extlink&oid=123466577&loc=SL123495735-112612

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1D
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123466505&loc=SL123495340-112611

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=gern_HerculesAndSiliconValleyBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=gern_SecondAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_TrancheFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_OptionOneMinCashDebtCovenantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_OptionTwoMinCashDebtCovenantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=gern_OptionThreeMinCashDebtCovenantMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

DEBT - Additional Information (Details) - USD ($)		6 Months Ended
DEBT - Additional Information (Details) - USD ($)	Jun. 01, 2022	Jun. 30, 2022	Sep. 30, 2020
Debt Instrument [Line Items]
Unamortized debt discount and issuance costs		$ 801,000
Hercules and Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		125,000,000	$ 75,000,000
Principal amount outstanding under term loan		50,000,000
Carrying value of term loan, net		50,420,000
Unamortized debt discount and issuance costs		801,000
Accrued end of term charge		1,221,000
Hercules and Silicon Valley Bank [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Available remaining loan principal under second amendment		75,000,000
Debt discount under second amendment		100,000
Debt issuance costs		$ 14,000
Prepayment charge (as a percentage)		1.50%
Charge for prepayment occurring 36 months after effective date		$ 0
Description of term loan payment terms		Under the second amendment, if we choose to prepay the principal with respect to any future draw down after the Effective Date, any such prepayment within the first 36 months after the Effective Date will be subject to a prepayment charge equal to 1.5% of the principal amount prepaid. No prepayment charge will be assessed for any prepayment occurring more than 36 months after the Effective Date.
Minimum cash balance as a percentage of loan amount outstanding	50.00%
Debt covenant minimum cash balance	$ 30,000,000
Hercules and Silicon Valley Bank [Member] \| Tranche One [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 20,000,000
Expiration date to borrow under a tranche for a debt instrument		Sep. 15, 2023
Hercules and Silicon Valley Bank [Member] \| Tranche Two [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 10,000,000
Start date to borrow under a tranche for a debt instrument		Jan. 01, 2023
Expiration date to borrow under a tranche for a debt instrument		Dec. 15, 2023
Hercules and Silicon Valley Bank [Member] \| Tranche Three [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 20,000,000
Start date to borrow under a tranche for a debt instrument		Sep. 15, 2023
Expiration date to borrow under a tranche for a debt instrument		Sep. 15, 2024
Hercules and Silicon Valley Bank [Member] \| Tranche Four [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under term loan		$ 25,000,000
Expiration date to borrow under a tranche for a debt instrument		Dec. 31, 2024
Hercules and Silicon Valley Bank [Member] \| Option One Minimum Cash Debt Covenant [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Minimum cash balance as a percentage of loan amount outstanding		40.00%
Hercules and Silicon Valley Bank [Member] \| Option Two Minimum Cash Debt Covenant [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Minimum cash balance as a percentage of loan amount outstanding		25.00%
Minimum market capitalization requirement for option		$ 750,000,000
Hercules and Silicon Valley Bank [Member] \| Option Three Minimum Cash Debt Covenant [Member] \| Second Amendment [Member]
Debt Instrument [Line Items]
Minimum cash balance requirement to be met as a percentage of six-month product revenue against forecast		70.00%
Minimum percentage of net product revenues maintenance period		6 months