You can remove Gemini recommendations from your email by going to Settings in Gmail → See all Settings → Uncheck "Smart Features" ("_Turn on smart features in Gmail, Chat, and Meet - When you turn this setting on, you agree to let Gmail, Chat, and Meet use your content and activity in these products to provide smart features and personalize your experience._"). You can also customize these settings by only unchecking certain features (e.g. Smart Reply, Smart Compose, Smart Compose personalization, etc.)