I had this problem too. A article from https://overhaulics.com/knowledgebase/post-smtp-token-has-been-expired-or-revoked-invalid_grant/ mentions that if you change your Google password then your OAuth will be denied. Thus you will need to grant permission again.
So I tried to Grant permission, but instead got either a 400 or 401 error from Google. A nice guy at OverHaulics assisted me and set up a key using Sendgrid API. So I did a form submission test twice last night, but I still don’t receive Gform submissions to my Gmail. However I do get the notification(we received your submission and will be in touch soon) from Gforms via sendgrid. So there’s some deeper more complicated issue at hand here.
Anyone who is a expert at this is welcome to contact me to resolve this plaguing problem that is costing us a lot of extra time, frustration and money.
Thnx – Brian