I'm slowly working my way through though. I will definitely use GnuPG and not RSA finally, for GnuPG is much more known. I'll just have to read the manual page many times.
The nice thing with OpenPGP messages (GnuPG is mainly a good implementation of the OpenPGP standard -
http://tools.ietf.org/html/rfc4880)
that you can extend the algorithms (check section 9.1 of the standard) used to make the signature while still using the same message format.
Don't forget that is depending of the OpenPGP private key used you might use RSA as signing algorithm as the key might be in RSA...
Kind regards,
PS: Thank you for the BC donation to Forban.