Fair value of interest rate swap contract.

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of gain loss on adjustment for fair value for convertible promissory notes.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Net gain loss to change in fair value.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Number of promissory notes issued.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	duration

- Definition

Threshold number of business days following a qualified financing.

+ References

No definition available.

+ Details

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Namespace Prefix:	gls_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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Balance Type:	na
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

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Balance Type:	credit
Period Type:	instant

- Definition

The amount for notes payable (written promise to pay), due to related parties. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Balance Type:	credit
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- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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- Details

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- Details

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- Details

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- Details

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- Details

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Debt - Additional Information (Details) $ in Millions			3 Months Ended	9 Months Ended
Debt - Additional Information (Details) $ in Millions	Aug. 04, 2022 USD ($) Days Notes	Jan. 19, 2022 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 13, 2021 USD ($)
2021 Bridge Financing
Debt Instrument [Line Items]
Principal amount							$ 27.0
Notes payable							$ 27.1
Loss on change in fair value of the convertible promissory notes				$ (0.2)
Repayment of debt		$ 27.3
2022 Promissory Notes
Debt Instrument [Line Items]
Notes payable			$ 25.9	$ 25.9
Loss on change in fair value of the convertible promissory notes			(0.8)
CMS Bridging DMCC, PureTech Health LLC and SSD2 LLC \| 2022 Promissory Notes
Debt Instrument [Line Items]
Number of promissory notes issued \| Notes	3
Aggregate principal amount of notes	$ 25.0
Aggregate cash purchase price	$ 25.0
SSD2 LLC \| 2022 Promissory Notes
Debt Instrument [Line Items]
Interest rate	15.00%
Maturity date	Dec. 31, 2023
Threshold number of business days following a qualified financing \| Days	5
Interest Rate Swap Contract on Intesa Sanpaolo Loan 1
Debt Instrument [Line Items]
Maturity date				Oct. 31, 2029
Net gain loss to change in fair value				$ 0.8	$ 0.1
Interest Rate Swap Contract on Intesa Sanpaolo Loan 1 \| Other Assets
Debt Instrument [Line Items]
Fair value of interest rate swap contract			$ 0.7	$ 0.7
Interest Rate Swap Contract on Intesa Sanpaolo Loan 1 \| Other Long-Term Liabilities
Debt Instrument [Line Items]
Fair value of interest rate swap contract						$ 0.1