An Acronym for Gnu Privacy Guard.
GNU's GPL re-implementation of PGP, initiated after the original went commercial and closed source. The package is called "gnupg" but the binary executable program is called gpg(1). It is widely used in OpenSource infrastructure for signing software packages (Debs and RPMs). Also widely used for signing and encrypting mail.
See also
Part of CategoryCryptography
29 pages link to GPG: