Try to define SSL inspection exclusions for Google's sites like https://accounts.google.com and others. I think Google checks for certificate validity by itself. Unfortunately I wasn't able to find a pre-defined category for this kind of URLs.
Adding an exception in XG firewall requires you to choose a Web Category or add a new one. I was trying to add a new one for Google Drive but you can't add a RegEx (just like in UTM9) to add sites like *.docs.google.com to the domains.