You could build a Chrome (or Firefox) extension that added GPG/PGP/SMIME to Gmail, you would have to intercept the emails before they were stored as drafts in order to protect the message in the inbox. You could use a plugin or native client to interface with the OS or desktop environment's keystore to keep the private key out of Javascript.
The key passphrase could double as the passphrase for symmetrically encrypting the message stored in the inbox.
Add to this a keyserver for automatically discovering public keys of contacts and you have a "good" solution between interested parties, without compromising recoverability of the majority of your messages.
You could do the same for Gtalk/Hangouts chats with OTR.
Add to this a keyserver for automatically discovering public keys of contacts and you have a "good" solution between interested parties, without compromising recoverability of the majority of your messages.
You could do the same for Gtalk/Hangouts chats with OTR.