It might be a little tricky in the case of GPG since it is really a GPL port of the commercial PGP software. So a major reason for it's original existence is that it is GPL code.
Gpg is not a port of some PGP version but an implementation of rfc4880 and earlier open standards. I actually took great care not to look at any "open source" PGP code. The reasons why dual-licensing does not work is that I assigned the copyright to the FSF in 1998 and when I terminated that contract 2 years ago too many other other hackers have code in it or assigned it to the FSF. Thus there is no way for anyone to switch to a dual-license.
It might be a little tricky in the case of GPG since it is really a GPL port of the commercial PGP software. So a major reason for it's original existence is that it is GPL code.