I moved to Fastmail from gmail a few years ago. I'm thrilled to not have Google searching my primary email address any longer. I still use gmail for SSO for a lot of things from my 'gmail.com' account.
You may already know this, but in case anyone else doesn't, you can create a Google account using a non-Google email address, so your SSO doesn't have to depend on a Gmail address if you don't want
