To see the permissions that have been granted on your Google Account just browse to http://accounts.google.com and once you are logged in look under Sign-in & security and click on Connected apps & sites.
Scroll down this page until you find Connected apps & sites then click on Manage Apps
Click on the entry labeled Pokemon Go Release and click the Remove button.
A confirmation dialog will pop up to confirm you want to revoke access to the app and that permission will be removed when you click OK.