Can a human read your Gemini chats?

Yes, and Google says so in plain words:

• Trained reviewers, including people at service providers, read and annotate a sample of conversations.
• Google’s own warning: "Please don’t enter confidential information that you wouldn’t want a reviewer to see or Google to use to improve our services, including machine-learning technologies."
• The setting that controls this is now called Keep Activity, renamed from Gemini Apps Activity.
• Turn Keep Activity off and chats are still held about 72 hours to run the service. Leave it on and activity auto-deletes at 18 months by default, adjustable to 3 or 36 months, or never.
• Reviewed chats are the catch: once a conversation is sampled for human review, it is disconnected from your account and kept separately for up to three years, and deleting your chat does not pull it back.
• Temporary chats are the exception: Google says they are not used to train its AI models or improve its AI with help from human reviewers.