Document and Entity Information
| Document and Entity Information | Jun. 09, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Jun. 9, 2022 |
| Entity Registrant Name | GTJ REIT, INC. |
| Entity Central Index Key | 0001368757 |
| Entity File Number | 333-136110 |
| Entity Incorporation, State or Country Code | MD |
| Entity Tax Identification Number | 20-5188065 |
| Entity Address, Address Line One | 1399 Franklin Avenue |
| Entity Address, Address Line Two | Suite 100 |
| Entity Address, City or Town | Garden City |
| Entity Address, State or Province | NY |
| Entity Address, Postal Zip Code | 11530 |
| City Area Code | 516 |
| Local Phone Number | 693-5500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |