I ran into the same problem, I need to have an email after I got access to GmailScopes.GMAIL_SEND.

authorizationResult.toGoogleSignInAccount() always returns null

I spent a lot of time poking around and found a way to get an email, not sure how correct it is:

After authorizing access to the desired scope, call:

new GoogleAccountManager(App.getInstance()).getAccounts()[0].name