Hi Colin,
Check the Google admin panel SSO settings to see if the login address was changed.
My school district has been using GSaml since 1.9. We are now using it with 2.6. It is working for us, but yesterday a setting had been changed on the Google end that caused it to break for a few hours. We do not use https on our webserver. Somehow, the SSO login address on the Google side was changed from http to https. After that, users were getting an SSL connection error.
It took a few hours to figure out what had happened. After reverting the address back to http, the SSO functioned as expected. The weird thing is there is no record of the SSO address getting switched in the first place. I checked the admin logs in Google and can see where I reverted the address, but the switch to https is nowhere to be found.
Please post back if this is what happened to you. I have a ticket out with their tech support. It would be helpful if I can show that it happened to more than just my district.