I just received stadia as a birthday gift I tried to log in with my email instead of the email that it was purchased with & it’s asking me for my billing information. I’m not trying to input it again & get double billed.
You shouldn't get double-charged if you enter your billing info. Stadia is free; you just pay for the games.
If you are trying to activate a Stadia Pro free trial, you will get a free month and then start to be billed $10 monthly unless you cancel. Stadia Pro gives you access to a cumulative library of games and better streaming quality, but it is not required to play games that you have already purchased.