You want to send and receive encrypted email conveniently with the elm mailer.
While viewing an encrypted message, type:
| gpg --decrypt | less
to display the decrypted text page by page. To send an encrypted message, encrypt it in your text editor. [Recipe 8.1][Recipe 8.2]
We take advantage of elm's pipe feature, which sends the body of a mail message to another Linux command, in this case gpg. We further pipe it to a pager (we chose less) for convenient display. For encryption, we handle it in the text editor invoked by elm to compose messages. [Recipe 8.1][Recipe 8.2]
There are alternatives. A patched version of elm, known as ELMME+ , supports GnuPG directly. (The author, Michael Elkins, went on to create mutt, [Recipe 8.6] which also supports GnuPG.)
You might also try the pair of scripts morepgp (for decrypting and reading) and mailpgp (for encrypting and sending), available at http://www.math.fu-berlin.de/~guckes/elm/scripts/elm.pgp.scripts.html. These scripts are for PGP, but modification for GnuPG should not be difficult.
The elm home page is http://www.instinct.org/elm. Read more about the scripts morepgp and mailpgp at http://www.math.fu-berlin.de/~guckes/elm/scripts/elm.pgp.scripts.html and http://www.math.fu-berlin.de/~guckes/elm/elm.index.html#security.