Woah, I never even knew about that. I was using GDevelop's default leaderboard system and it never had a sign in prompt like that before. I don't know why it did that. Sorry for the inconvenience.
Edit: I'm going to assume that you maybe clicked the claim your score button in the leaderboard. If it's that, I'm pretty sure that requires a GDevelop account which is why it's asking for your email. I have no idea why it made your username your password but that's out of my control, and your gonna have to report it to the GDevelop devs themselves. Again, sorry for trouble.