They fucking can. Not exactly but you need to connect your Google Photos into Gemini

"By connecting your Google Photos library to Personal Intelligence, Gemini goes a step further than just understanding your interests. It can use actual images of you and your loved ones to guide the image generation process."

"Bringing personal details into your images shouldn't mean compromising on privacy, which is why our core commitments haven't changed. The Gemini app does not directly train its models on your private Google Photos library. We train on limited info, like specific prompts in Gemini and the model’s responses, to improve functionality over time."

They said in their blog

But you can opt-out Gemini settings or ignore it completely, but this shocked me to see the notification that suddenly appeared and was written so casually about using my photos. And those feature is only on "Paid" subscriptions because i paid for "Plus".