Which meetings get a brief?
Any event with at least one other person, unless it is all-day, you declined it, or its title is on your ignore list. Personal events get a deliberately thin brief unless you route them elsewhere.
What does the model actually read?
The calendar event; recent email threads with the guests; files shared with or by them, or matching the meeting; your past meetings with the same people; and public web pages about the company and the attendees. Every source is listed on the review page, and what could not be verified is marked.
Can guests tell it was generated?
The footer says Preread. The brief is written to be read, not to impress, and you have approved it.
What if it gets something wrong?
Edit the prompt and rerun, or skip. Wrong facts tend to come from wrong sources, which is why the sources are listed. A brief that could not be drafted is marked failed with the reason.
Does it work with Outlook?
Not yet. Google Workspace and Gmail only, for now.
Where does the data live?
Briefs, prompts and the selector are stored, encrypted, with your account on Google Cloud. Retrieved mail and files are stored encrypted only for as long as your retention setting says. Details on the security page.