I'm sure Simon will answer this more fully, but (basically) OpenPGP is a "standard" and GPG is just one "OpenPGP Compliant" implementation (there are others). The idea is that an OpenPGP compliant app should be able to handle ciphertext from any of the others.
So we have OpenPGP too? Is that different to PGP? How is it different to GPG lol - *head on desk*
Thanks for the link!