latest versions of iOS are not working well with GPG, sadly. For most users it's working fine, but for some, and for reasons I have yet to find, it crashes.
First of all, you *can* backup your progress without the cloud. You can export to a file, and then use iTunes File Sharing to copy it to your PC. then you can similarly copy it to your other device if needed.
Regarding the cloud crash: if you own an android device, try to open the Google Play Games app and create a new gamer ID. That has helped some users with the issue.