Is it there a way to make duplicity backups to Google Drive?
As far as I understand, WebDav and FTP are not supported by Google Drive. But maybe there is an extension to duplicity in the works?
The only way (IMO) would be to configure a Windows system with Google Drive (there's no Linux client yet, not sure about OSX), share the Google Drive folder, and then mount it on your target *nix system with Samba and use it as a backup target.
It would be convoluted, prone to errors and breakage, and probably perform pretty badly. But you could, if you really had to.
There used to be a plugin for GoogleDocs (announcement), but that one seems to have stopped working at the time GoogleDrive was introduced. I shortly looked into it, and it seems to me that some of the Google GData client libraries (which the plugin uses) still need to be updated.
This guy seems to have it figured out. When I tried on Ubuntu 11.10, I had an earlier version of duplicity than what he uses. I updated to v0.6.19, and it works like a charm.