Related party transactions - Disclosure of Liabilities to Related Parties (Details) - EUR (€) € in Thousands	Mar. 31, 2024	Mar. 31, 2023
Liabilities to key management personnel:
Net defined benefit liability (asset)	€ 4,587	€ 3,327
Liabilities to shareholders:
Loans and borrowings	588,867	788,836
Warrants liabilities	13,689	19,227
Closing balance for the year	657	65,431
Key management personnel
Liabilities to key management personnel:
Net defined benefit liability (asset)	390	196
Shareholders
Liabilities to shareholders:
Loans and borrowings	0	61,324
Shareholders \| Warrants liabilities
Liabilities to shareholders:
Warrants liabilities	€ 267	€ 3,911

Liabilities To Key Management Personnel

No definition available.

Name:	gb_LiabilitiesToKeyManagementPersonnelAbstract
Namespace Prefix:	gb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Liabilities To Shareholders

+ References

No definition available.

+ Details

Name:	gb_LiabilitiesToShareholdersAbstract
Namespace Prefix:	gb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amounts payable resulting from related party transactions. [Refer: Related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2023-01-01
-Paragraph 20
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=24&code=ifrs-tx-2023-en-r&anchor=para_20&doctype=Standard
-URIDate 2023-03-23

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2023-01-01
-Paragraph 18
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=24&code=ifrs-tx-2023-en-r&anchor=para_18_b&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_AmountsPayableRelatedPartyTransactions
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of outstanding funds that the entity is obligated to repay.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2023-01-01
-Paragraph 55
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2023-en-r&anchor=para_55&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_Borrowings
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of current derivative financial liabilities. [Refer: Derivative financial liabilities]

+ References

+ Details

Name:	ifrs-full_CurrentDerivativeFinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
-Paragraph 140
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2023-en-r&anchor=para_140_a&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_LiabilityAssetOfDefinedBenefitPlans
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	ifrs-full_CategoriesOfRelatedPartiesAxis=ifrs-full_KeyManagementPersonnelOfEntityOrParentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CategoriesOfRelatedPartiesAxis=gb_ShareholdersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialLiabilitiesAxis=gb_WarrantLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: