What needs to be done for this to not have the Google requirements?
Current goals are to vet the idea of using DNS-01 challenges and get the UX right.
Also, lego is a bit cumbersome to run in a container. It relies on storing config and certs in a .lego dir in the home directory, and doesn't clean up DNS entries if it somehow fails to run so it would be a pain to use without modification.
I think they should be possible to support with no or minimal changes to the API you have in the GCE provider currently. Mostly just need to make changes around the service-account/secret handling, I believe.