gpg is the main command line tool of the GNU Privacy Guard (GnuPG), implementing the OpenPGP standard for public key cryptography. It allows encryption, (verification of) signatures and trust models like the web of trust. gpg (and its library gpgme) are the base of many FOSS crypto applications (like mail user agents). There is a standalone version (1.x) and one (2.x) requiring gpg-agent for secret key handling.
GNU implementation of OpenPGP, the cryptographic toolset : http://gnupg.org/
List of GUIs: https://www.gnupg.org/related_software/frontends.html