PGP 5.x: This version of PGP is formerly known as “PGP 3” in the community.
PGP - Pretty Good Privacy. PGP is a family of software systems developed by Philip R. Zimmermann from which OpenPGP is based.
GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP implementation that avoids all encumbered algorithms. Consequently, early versions of GnuPG did not include RSA public keys. GnuPG may or may not have (depending on version) support for IDEA or other encumbered(restrict in such a way that free action or movement is difficult.) algorithms.
With the advancement of cryptography, parts of PGP have been criticized for being dated: