When you sign into Morty using Google, Google will usually prompt you to choose the account you'd like to use, then you can choose the one you used to sign up for Morty.
However, if you are signed into only one Google account and it isn't the account you use with Morty your login might fail because Google will try to use that account by default.
To get Google to sign in using your intended Google account:
Open your default browser (e.g. Chrome or Safari) and go to Google.com
Tap your profile picture in the right corner to open the account menu
Sign into the Google account you used to sign up for Morty
Open Morty, Sign in using Google and choose the correct account!
That's it! 🎉