Google Mail / "GMail" has no fewer than nine settings for "Smart Features and Personalization", e.g., Gemini. I have these all turned off in order to prevent Gemini ingesting my emails. To do this, you now have to opt out of benign machine-learning or even pattern-matching-based features, like sorting promotional emails from regular ones or spellcheck.

Despite this, as of today, Google is offering a Gemini smart compose feature on all my emails, despite me having turned off smart compose.