Interesting, but can't move past the sign-in portion. I'm not giving a website the ability to see my message subjects, headers and labels. Nothing on your site landing page convinces me of the value in doing so.
Sign in with username/password, then. Signal doesn't require Gmail. You can add your Gmail later if you want to / see the benefit to you in doing so. Signal's biggest benefit to founders right now is the investor searching/finding capabilities. You don't need to be Gmail authed to get value from that. If you build, say, a target list of 50+ investors in Signal, you then might want to see your strongest intro paths to those investors. That's when you can consider the Gmail auth. Always up to you.