GPG, or GNU Privacy Guard, offers a free alternative to the now-commercial PGP. GPG is an implementation of the OpenPGP standard, and as such is quite similar to PGP, with the biggest difference being that it is openly available and supported strictly by donations. Like PGP, GPG lets users encrypt and sign data and emails. It works as a command line tool, and as such is easy to integrate with a variety of applications, though the lack of a more graphical interface may make it even more difficult to use than the commercial PGP.
source:
http://www.ehow.com/facts_7201163_pgp-encryption.html