Subsequent events - Share Consolidation (Details) - Share Consolidation	Feb. 01, 2021 shares	Jan. 12, 2021
Disclosure of non-adjusting events after reporting period [line items]
Share consolidation ratio		0.20
Reduction in number of issued Common Shares	67,500,000
Reduction in number of outstanding Common Shares	67,500,000
Common shares, issued	13,500,000
Common shares, outstanding	13,500,000
Number of fractional shares issued	0

Number of fractional shares issued during period.

No definition available.

Name:	gbnh_NumberOfFractionalSharesIssuedDuringPeriod
Namespace Prefix:	gbnh_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Ratio applied to the consolidation of stock, for example but not limited to, one share converted to two or two shares converted to one.

+ References

No definition available.

+ Details

Name:	gbnh_ShareConsolidationRatio
Namespace Prefix:	gbnh_
Data Type:	xbrli:pureItemType
Balance Type:	na
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

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

- Definition

The increase (decrease) in the number of ordinary shares issued. [Refer: Ordinary shares [member]]

+ 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

Name:	ifrs-full_IncreaseDecreaseInNumberOfOrdinarySharesIssued
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The increase (decrease) in the number of shares outstanding. [Refer: Number of shares outstanding]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 79
-Subparagraph a
-Clause iv
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_79_a_iv&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_IncreaseDecreaseInNumberOfSharesOutstanding
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares issued by the entity.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 106
-Subparagraph d
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_106_d&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_NumberOfSharesIssued
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares that have been authorised and issued, reduced by treasury shares held. [Refer: Treasury shares]

+ References

+ Details

Name:	ifrs-full_NumberOfSharesOutstanding
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	ifrs-full_NonadjustingEventsAfterReportingPeriodAxis=gbnh_ShareConsolidationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: