This seems like a fantastic idea. We currently use Syncdocs [1] to encrypt our Google Drive folders. It is also end-to-end encryption, but it uses AES, not PGP, which means the key exchange is separate.

If the email is drafted online, do the drafts get deleted and wiped after encryption on the Gmail server?

[1] http://www.syncdocs.com/google-drive-encryption-faq/