Q: Backing up photos to iCloud Photo Library on mac

See if this helps -- http://www.imore.com/how-to-move-files-to-icloud-drive-macos-sierra

OR this -- http://www.picbackman.com/tips-tricks/how-to-migrate-google-drive-photos-and-vid eos-to-icloud-drive/

You seem to want OSX Photos to monitor your Google Drive folder, and update your iCloud Photo Library as it goes.

Is that right?

This is not a default process for sure. Nothing native or built-in as OSX imports photos manually or via iCloud.

Check this option >>> https://photosautomation.com/import-hot-folder.html

Automator is a standard OSX app. I have used this for a local hot folder, but not with a cloud based folder.

If Google drive is actually storing your photos locally, this should work.