Google-Extended is not a separate crawler with its own user agent; it is a control you set in robots.txt to opt your content in or out of Google's AI training and grounding uses.
Because it is decoupled from Search ranking, blocking Google-Extended does not protect or harm your blue-link rankings. AI Overviews are fed by ordinary Googlebot, not Google-Extended, so blocking the token does not remove you from AI Overviews either. It governs Gemini training and grounding in the Gemini app and the API, so weigh AI visibility against training concerns when deciding. Our Gemini SEO guide maps which Gemini surfaces the token actually gates.