Google's Gemini trains on chat history, including personal accounts with subscriptions, unless users turn off chat history or use temporary chat sessions. This data may be reviewed by human reviewers and used to improve Google services. Users are warned not to enter confidential information, as it may be retained for up to three years. To disable training on chat sessions, users must turn off 'Keep Activity' in Gemini's settings, which means losing chat sessions after 72 hours.