The signals aren't documented (for obvious reasons), but I'd be surprised if Google Analytics were a signal. These things are usually kept separate, and Analytics is a lot less user-specific under GDPR as the anonymizeIP flag is now very common.

That said, I've no evidence one way or the other!

My understanding is that it comes down to information they can read about your browser (does this look like a bot environment?), and heuristically how the user has behaved since the JS has been loaded (mouse movements, time between actions, etc).