So you apparently migrated your Nest Account to a Google Account as described here: https://support.google.com/googlenest/answer/9297676?hl=en
Are you able to sign into the Google Nest app using the "Sign in with Google" button, using the Google (gmail) account and password you migrated to?
What happens when you try to sign into the Google Home app using the Google (gmail) account and password you migrated to?
Seems like he has the same issue I have. The nest app on my sony google smart tv does not give u the option to sign in via Google. It only shows the sign in via the nest account. Error message displayed is when u try to sign in with your Google credentials. It says username or password incorrect. The nest app for the Google tv does not have a Google migration button or a sign in via google button. How can somebody sign in then?
I'm just another Google Nest customer, sharing the following, which doesn't seem to make sense, since Google Nest seems to want everyone to migrate to Google:
This Google Nest Help page says that for an Android TV, you need "A Nest Account that hasn’t been migrated to a Google Account."
This Google Nest Help page says "Once you migrate your account, you’ll no longer be able to use the Nest app on Apple TV or on Android TV.":