How to fix 'too many redirects' error while logging in?

When logging into your Google account on merchantmail.app, people sometimes see an error like "script.google.com redirected you too many times". This is due to a bug in Google's login system.

Google login too many redirects error
Google login too many redirects error.

Try the following steps to fix the issue. Note that you only need to do this once, when completing the authorization steps for the Gmail Add-on. Once that's done, you won't need to use incognito mode.