Reset my phone and now I can't remember my Gmail password

Not sure which Gmail was on the phone? Sign in to any account you think it might be on a computer, go to Security > Your devices, and see if the phone is listed. That tells you which one it's waiting for.

So the phone is now asking for the Google account that was on it before the reset? That's Factory Reset Protection, and it'll only accept that same account. Do the password reset from a computer or another phone: Google sign-in, enter your Gmail address, Forgot password. If your phone number is on the account, a text code is the quickest way through. Put the SIM in another phone if needed. Once the password is changed, give it a little while before using it on the reset phone, because a freshly changed password can be refused there for a short time.