Mac Mail El Capitan - One gmail account login failed
Just updated an older Mac (2009 model) to El Capitan (updated to Yosemite previously and all was fine then). In Mac Mail - One of my gmail accounts login failed (one other gmail account routed through Mac Mail is just fine). Have tried in settings unchecking activation of the mail app box, rebooting and trying to reactivate and no change. Apple support pages keep asking to verify password is correct but I cannot see a password anywhere in the settings box where illustrated in other articles (it it supposed to be in the "account information" in system preferences, under "full name" but it is not for me).
iMac, OS X El Capitan (10.11.3)