[Answer]-Gmail api forbidden error with using the access token

According to the Google APIs Explorer, the OAtuth 2.0 scope you’re using:

https://www.googleapis.com/auth/gmail.compose

… only allows:

Manage drafts and send emails

But you’re trying to use it for this:

https://www.googleapis.com/gmail/v1/users/me/messages

That is, to view email. You need the right scope:

https://mail.google.com/

… which allows:

View and manage your mail

So the error message is completely appropriate. You’ve asked for and gotten permission to do one thing, tried to do something different, and gotten a permissions error.