Other answers will help to disable Gemini on a project level.

If you wish to disable Gemini entirely: At time of writing, as of Android Studio Meerkat, Gemini features are provided in a plugin called "Gemini". You can disable the plugin from Settings > Plugins:

So far, I've found no trace of Gemini features in my editor or toolbars after disabling the plugin. The same works for GitHub integrations and other built-in functions, in case you wish to disable those entirely as well.