Ok, after some more searching I've found a newer cert here.. https://pki.google.com/ which expires at the beginning of 2018 and installed it in the proper cert directory (intermediate certificates) but The Bat still gives me the same message about an expired cert. I tried deleting the older but identical google/geo trust cert but that made no difference.
Solved the problem. There is a setting in the bat, options>S/MIME and TLS, to use internally stored certs or Windows certs. It was set to internal so I switched it to Windows and now it's working. Never knew that was an option and it's not well documented.
Very strange that this should now be an issue. I am using 18.104.22.168 and had the same problem with the same certificate. Switched to "Microsoft CryptoAPI (Windows Certificate Store)" and finally am receiving emails again from my gmail accounts.