If you want to log on to AutoProctor with a different gmail account, you need to Logout first. For this:

1. On Mobile: Click on the three horizontal bars at the top left of your screen and then click on Logout

2. On Desktop: Click on your name at the top right of your screen where it says "Hi .." and then click on Logout 

If that still doesn't work, log out of all your Google Accounts by visiting https://accounts.google.com/Logout And then log on to AutoProctor again.