Google Identity Services (GIS) is Google's set of client libraries for identity on the web. It covers authentication, which establishes who a user is, through Sign in with Google, and authorization, which gives an app an access token for Google APIs and the user's data. GIS is based on OAuth 2.0, the standard protocol for authorization, and Google says it aims to be easier and more secure for developers than OAuth and OpenID Connect used directly.
A developer adds GIS elements to a site, with the HTML API and its code generator or with the JavaScript library: a personalized Sign in with Google button, the One Tap prompt and automatic sign-in on return visits. For authorization, GIS starts OAuth flows in browser pop-up dialogs and returns an access token or an authorization code, with separate flows for sign-in and for consent. The JavaScript library runs only in browsers. GIS includes CSRF protection (a defense against forged requests from other sites) by default and works with the browser's credential manager APIs.
Google states that data from Sign in with Google is not used for ads. The pages read state no price.