Gladstone Investment Corporation Quarterly report pursuant to Section 13 or 15(d)

Quarterly report pursuant to Section 13 or 15(d)

Cover
- Cover
Financial Statements
Notes to Financial Statements
Accounting Policies
- SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)
Notes Tables
Notes Details

SUBSEQUENT EVENTS - Narrative (Details)

v3.24.0.1

SUBSEQUENT EVENTS - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 24, 2024	Jan. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Feb. 05, 2024	Feb. 04, 2024	Oct. 30, 2023	Oct. 29, 2023	Mar. 31, 2023
Adviser \| David Gladstone
Subsequent Event [Line Items]
Ownership percentage by parent			100.00%		100.00%
Adviser \| Gladstone Companies, Inc
Subsequent Event [Line Items]
Ownership percentage by parent			100.00%		100.00%
Gladstone Companies, Ltd. \| David Gladstone
Subsequent Event [Line Items]
Ownership percentage by parent			100.00%		100.00%
Gladstone Companies, Inc \| Gladstone Companies, Ltd.
Subsequent Event [Line Items]
Ownership percentage by parent			100.00%		100.00%
Credit Facility \| Revolving Line of Credit \| Line of Credit
Subsequent Event [Line Items]
Commitment amount			$ 135,000		$ 135,000				$ 135,000	$ 180,000	$ 180,000
Common Stock ATM Program
Subsequent Event [Line Items]
Sale of stock, number of shares sold (in shares)			1,456,279	212,338	1,760,449	241,978
Sale of stock, weighted-average gross price per share (in USD per share)			$ 14.51	$ 14.11	$ 14.34	$ 14.31
Sale of stock, consideration received			$ 21,100	$ 3,000	$ 25,300	$ 3,500
Subsequent Event \| Adviser \| David Gladstone
Subsequent Event [Line Items]
Ownership percentage by parent	100.00%
Subsequent Event \| Adviser \| Voting Trust Board
Subsequent Event [Line Items]
Ownership percentage of voting rights	20.00%
Effective period after agreement is entered into	1 year
Subsequent Event \| Credit Facility \| Revolving Line of Credit \| Line of Credit
Subsequent Event [Line Items]
Commitment amount							$ 200,000	$ 135,000
Subsequent Event \| Common Stock ATM Program
Subsequent Event [Line Items]
Sale of stock, number of shares sold (in shares)		538,206
Sale of stock, weighted-average gross price per share (in USD per share)		$ 14.53
Sale of stock, consideration received		$ 7,700

- Definition

Effective Period After Agreement Is Entered Into

+ References

No definition available.

+ Details

Name:	gain_EffectivePeriodAfterAgreementIsEnteredInto
Namespace Prefix:	gain_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Ownership Percentage Of Voting Rights

+ References

No definition available.

+ Details

Name:	gain_OwnershipPercentageOfVotingRights
Namespace Prefix:	gain_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Sale Of Stock, Weighted-Average Gross Price Per Share

+ References

No definition available.

+ Details

Name:	gain_SaleOfStockWeightedAverageGrossPricePerShare
Namespace Prefix:	gain_
Data Type:	dtr-types1:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockConsiderationReceivedOnTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 830
-SubTopic 30
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481674/830-30-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 855
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483399/855-10-50-2

+ Details

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_OwnershipAxis=gain_TheAdviserMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gain_DavidGladstoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gain_GladstoneCompaniesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=gain_GladstoneCompaniesLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=gain_GladstoneCompaniesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gain_GladstoneCompaniesLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=gain_RevolvingLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=gain_CommonStockATMProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=gain_VotingTrustBoardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: