Employee benefit obligations - Disclosure of Income Statement Impact (Details) - EUR (€) € in Thousands	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2023
Disclosure of net defined benefit liability (asset) [line items]
Total net periodic cost	€ 1,649	€ 1,365	€ 1,607
Pension defined benefit plan
Disclosure of net defined benefit liability (asset) [line items]
Current service cost	1,394	1,169	1,088
Interest cost	213	95	74
Past service cost	141	82	519
Unrealized FX impact	37	152	(93)
Total net periodic cost	€ 1,785	€ 1,498	€ 1,588

Gains (Losses) Arising From Unrealized Foreign Exchanges, Defined Benefit Plans

No definition available.

Name:	gb_GainsLossesArisingFromUnrealizedForeignExchangesDefinedBenefitPlans
Namespace Prefix:	gb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The expense of employee service in the current period arising from defined benefit plans. [Refer: Post-employment benefit expense in profit or loss, defined benefit plans] [Contrast: Increase in net defined benefit liability (asset) resulting from current service cost]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 135
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_135_b&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_CurrentServiceCostDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfNetDefinedBenefitLiabilityAssetLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of interest expense (income) arising from defined benefit plans. [Refer: Interest income (expense); Post-employment benefit expense in profit or loss, defined benefit plans] [Contrast: Increase (decrease) in net defined benefit liability (asset) resulting from interest expense (income)]

+ References

+ Details

Name:	ifrs-full_InterestExpenseIncomeDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of expense (income) resulting from past service cost. Past service cost is the change in the present value of the defined benefit obligation for employee service in prior periods, resulting from a plan amendment (the introduction or withdrawal of, or changes to, a defined benefit plan) or a curtailment (a significant reduction by the entity in the number of employees covered by a plan). [Refer: Post-employment benefit expense in profit or loss, defined benefit plans; Defined benefit plans [domain]] [Contrast: Increase (decrease) in net defined benefit liability (asset) resulting from past service cost]

+ References

+ Details

Name:	ifrs-full_PastServiceCostDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of post-employment benefit expense included in profit or loss relating to defined benefit plans. [Refer: Profit (loss); Defined benefit plans [domain]] [Contrast: Increase (decrease) in net defined benefit liability (asset) resulting from expense (income) in profit or loss]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 5
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_5&doctype=Standard
-URIDate 2024-03-27

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2024-01-01
-Paragraph 135
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2024-en-r&anchor=para_135_b&doctype=Standard
-URIDate 2024-03-27

+ Details

Name:	ifrs-full_PostemploymentBenefitExpenseDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	ifrs-full_CharacteristicsOfDefinedBenefitPlansAxis=ifrs-full_PensionDefinedBenefitPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: