If you find yourself in this predicament, this is how you do it. It’s a two-step process to fully remove GMass from your Chrome browser and from your Gmail account.
Remove GMass from your browser by navigating Chrome to chrome://extensions, finding GMass in the list of extensions, and either unchecking the Enabled box it or deleting it entirely with the trash can icon.
Disconnect GMass from your Gmail account. Go to Apps Connected to your Google Account, and find GMass, select it, and click REMOVE.