Clear the checkboxes for: Smart features in Google Workspace, Smart features in other Google products
If you have more Gmail accounts, repeat these steps for each one.
Turning off Gemini in Gmail also disables basic, long-standing features like spellchecking, which predate AI assistants. This design choice discourages opting out and shows how valuable your AI-processed data is for Google.
This has finally gotten me to take steps to deGoogle my email, Fastmail trial underway.