Exhibit 5.1

[Letterhead of Sullivan & Cromwell LLP]

November 20, 2025

Gartner, Inc.,

56 Top Gallant Road,

Stamford, CT 06902.

Ladies and Gentlemen:

In connection with the registration under the Securities Act of 1933 (the “Act”) of (i) $350,000,000 aggregate principal amount of 4.950% Senior Notes due 2031 (the “2031 Notes”) of Gartner, Inc., a Delaware corporation (the “Company”), issued pursuant to the Indenture, dated as of November 20, 2025 (the “Base Indenture”), as supplemented by the First Supplemental Indenture, dated as of November 20, 2025 (the “First Supplemental Indenture”), each between the Company and U.S. Bank Trust Company, National Association, as trustee (the “Trustee”) and (ii) $450,000,000 aggregate principal amount of 5.600% Senior Notes due 2035 (the “2035 Notes” and, together with the 2031 Notes, the “Securities”) of the Company, issued pursuant to the Base Indenture, as supplemented by the Second Supplemental Indenture, dated as of November 20, 2025 (the “Second Supplemental Indenture” and, together with the Base Indenture and the First Supplemental Indenture, the “Indenture”), between the Company and the Trustee, we, as your counsel, have examined such corporate records, certificates and other documents, and such questions of law, as we have considered necessary or appropriate for the purposes of this opinion.

Upon the basis of such examination, we advise you that, in our opinion, the Securities constitute valid and legally binding obligations of the Company, subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and similar laws of general applicability relating to or affecting creditors’ rights and to general equity principles.

In rendering the foregoing opinion, we are expressing no opinion as to Federal or state laws relating to fraudulent transfers and we are not passing upon, and assume no responsibility for, any disclosure in any registration statement or any related prospectus or other offering material relating to the offer and sale of the Securities.

The foregoing opinion is limited to the Federal laws of the United States, the laws of the State of New York and the General Corporation Law of the State of Delaware, and we are expressing no opinion as to the effect of the laws of any other jurisdiction.

Gartner, Inc.		-2-

 

We have relied as to certain factual matters on information obtained from public officials, officers of the Company and other sources believed by us to be responsible, and we have assumed that the Indenture has been duly authorized, executed and delivered by the Trustee thereunder, an assumption which we have not independently verified.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to us under the heading “Validity of Securities” in the Prospectus Supplement relating to the Securities, dated November 13, 2025. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act.