That does not match what I see, gemini.google.com always present me a login page and it did the same for my colleague at work.
Are you and your colleague both trying at work? Probably on the same IP? Google might attribute less trust to an IP shared between many different users than it does to a regular residential internet IP (like mine).
Did some more testing and the behavior is interesting. When connecting through a Mullvad node in the US it doesn't require login, but any Mullvad node outside of the US and it does. I might be wrong its and it's just a per-country policy.