If you don't care about the license issues, eg. because you only use GnuPG for private mail and live outside the ``land of the free'', you can use the ability to load modules into GnuPG and to provide interoperability with PGP2 that way. SuSE provides the package gpgaddon which contains the idea and rsa modules and provides a wrapper script, called gpg-compat, which basically is exec gpg --load-extension rsa --load-extension idea -allow-non-selfsigned-uid $*. You may additionally want to use the option --rfc-1991. You can even import old pgp2 keyrings with the help of these options.
This simple approach will do most for you. However, it is not clever enough to produce encrypted messages readable by pgp2. For this, a perl script from Gero Treuner <email@example.com>, which is much more clever, is provided and put into /usr/bin/gpg-2comp.
The configuration of ggp-2comp is done via the config file ~/.gpg-2comprc. You need to set the key-ID of your RSA key there. Here is an example with comments stripped off.
# ------- gpg-2comp user configuration #$gpg = `/usr/bin/gpg-compat'; $use_pgp2 = 0; # YOU REALLY NEED TO SET THIS $rsa_default_key = `0x12345678'; $rsa_encryptself = 1; #$comment = `Scripting by http://muppet.faveve.uni-stuttgart.de/~gero/gpg-2comp';