Related party transactions - Purchases of services from related parties (Details) - EUR (€) € in Thousands	12 Months Ended
Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Disclosure of transactions between related parties [line items]
Monitoring fee (including Directors fee)	€ 218	€ 709	€ 776
Entities with significant influence over entity
Disclosure of transactions between related parties [line items]
Monitoring fee (including Directors fee)	(950)	460	700
Directors fee	278	195	0
Reimbursements	16	54	76
Total	€ (656)	€ 709	€ 776

Monitoring Fee

No definition available.

Name:	gb_MonitoringFee
Namespace Prefix:	gb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of remuneration paid or payable to the entity's directors.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2020-01-01

+ Details

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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 domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

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

- Definition

The amount of services received in related party transactions. [Refer: Related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 24
-IssueDate 2020-01-01
-Paragraph 21
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=24&code=ifrs-tx-2020-en-r&anchor=para_21_c&doctype=Standard
-URIDate 2020-01-01

+ Details

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

- Definition

The amount of liabilities settled on behalf of the entity by a related party in related party transactions. [Refer: Related parties [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 24
-IssueDate 2020-01-01
-Paragraph 21
-Subparagraph j
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=24&code=ifrs-tx-2020-en-r&anchor=para_21_j&doctype=Standard
-URIDate 2020-01-01

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Details

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