What is secure? If government sezies your key, in some jurisdictions they can compel you to release your passphrase (if one is required), then they can go back and decrypt absolutely everything you ever sent. (OTR gets around this but this is largely by leaving the network layer location privacy / initial authentication / key exchange problems out of scope.)
Finally, traffic analysis is pretty powerful. Even if Eve can't read your messages, she can see who they went to and when (and maybe who they in turn communicated with afterwards) and thus easily determine probable relationships - Tor based endpoint, or not. (Hrrm ... but two Tor based endpoints, on the other hand...)
"Secure" in this context means "not readable by anyone but the recipient". Of course, if you have the recipient in custody, this is the easiest method to decrypt the messages: http://xkcd.com/538/
(OTR-style cryptography is nice, but sucks for store-and-forward communication; for a comporably difficult message to decrypt more than once, use one-time pads - but don't complain to be about distributing the pad dictionary, that's totally out of scope here ;)
https://github.com/crised/SafeGmail
No good (edit: stable and drop-in) Firefox tool yet.
If you want sender/receiver privacy too, you can try using Tor hidden services direct to the individual. I can't think of any other way off the top of my head to send SMTP mail and completely conceal the recipient(s).