You can explore your Google Cloud's Application Storage part via Rsync, AFAIK. So you can see whether your backups are encrypted or not.

I remember that you had to extract at least two keys from the android device to be able to read "on-device" chat storage in the days of yore, so the tech is there.

If you don't have the keys' copies in the Google Drive side, we can say that they are at least "superficially" encrypted.