Welcome to the forum.
There is a link to the Schema (OpenAPI Specification v3.0.3) from this page in the Gemini documentation: مقدمه ای برای فراخوانی تابع با Gemini API | Google AI for Developers
If you scroll down that page, under the heading Function declarations the Google documentation now clarifies that the API implements a “select subset” of OpenAPI and they link to the subset (https://ai.google.dev/api/rest/v1beta/Tool#Schema, which is a broken link). In previous versions of the documentation Google had not specified that the implementation is partial and I had objected to that lack of disclosure in the documentation.
Hope that helps!