Jim uses Chromebooks, has three of them. But on his newest one, he keeps getting a notice to add Gmail to his Google Account, even though he already has one. Leo suggests that it may be time to "powerwash" it. Make sure all your local data is stored in the cloud and then select powerwash in the settings and relog in. You should never get that message again.
It could also be that Jim inadvertently created a new Google account and the Chromebook isn't savvy to his existing Gmail account. So try logging into your Gmail account when you turn on your Chromebook. See if that works. But Leo says to Powerwash it first and then log in using Gmail.