There is a wrapper program called pgpgpg which converts PGP 2.6 commands into GPG commands. You will need to get this and install it according to its directions.
Next you will need to get PGP4Pine, which is a set of filters you can add to pine to get PGP functionality. You must install this according to its directions, and then modify the filters so that they call pgpgpg instead of pgp.
By combing pine, PGP4Pine and pgpgpg, you should be able to use GPG with pine.
Now, you need to edit your /.pinerc file and change two lines:
display-filters=_BEGINNING("-----BEGIN PGP")_ /usr/lib/pinepgp/check _TMPFILE_ _RESULTFILE_
sending-filters=/usr/lib/pinepgp/pgp _TMPFILE_ _RECIPIENTS_
The path /usr/lib/pinepgp is what Debian uses for this package, which it calls pinepgp. On your computer, you may have PGP4Pine installed elsewhere; change the path names appropriately. This will allow you to receive and send messages which have been signed and/or encrypted with GPG.
To sign or encrypt a message in pine, compose the message as you normally would, and then when you get to this menu:
Send message (unfiltered)? _
Y [Yes] ^P Prev Filter
^C Cancel N No ^N Next Filter
Press Ctrl-N, and the menu will change:
Send message (filtered thru "pgp")? _
Y [Yes] ^P Prev Filter
^C Cancel N No ^N Next Filter
Pressing y brings up a separate PGP menu:
PINE/PGP send message --------------------- s - sign r - encrypt e - sign & encrypt n - do nothing f - finger address c - cancel [default = sign] Operation? _
From here, you can sign the message, or encrypt it, or do both. Even though it says PGP, it will actually use GPG, since you're translating PGP commands into GPG commands with pgpgpg.