| Document and Entity Information | Sep. 14, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | GARTNER INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000749251 |
| Document Type | 8-K |
| Document Period End Date | Sep. 14, 2020 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-14443 |
| Entity Tax Identification Number | 04-3099750 |
| Entity Address, Address Line One | P.O. Box 10212 |
| Entity Address, Address Line Two | 56 Top Gallant Road |
| Entity Address, City or Town | Stamford |
| Entity Address, State or Province | CT |
| Entity Address, Postal Zip Code | 06902-7700 |
| City Area Code | (203) |
| Local Phone Number | 316-1111 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.0005 per value per share |
| Trading Symbol | IT |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
