Typical Google forcing everything on you. I just tried logging into one of my AdSense accounts and the login email address has been a @companyname.com for 10+ years not a gmail account. Now I can't login into my account unless I create a gmail account and associate it with my AdSense account. Always an suspicious motive for why Google does these kinds of things. I also noticed today that you can be logged into your gmail and then click on the Adwords login. Now you have to verify the password (which for me is the same) whereas you didn't have to do this just yesterday. However they did not make this login verification step for Analytics. Always making their systems so their either harder to log out of or force you to use all things Google.