It can be considered as a backend for other applications.
Edit by : GnuPG itself is a commandline tool without any graphical stuff. It is the real crypto engine which can be used directly from a command prompt, from shell scripts or by other programs. Therefore it can be considered as a backend for other applications.

However, even when used on the command line it provides all functionality needed - this includes an interactive menu system. The set of commands of this tool will always be a superset of those provided by any frontends.

Features:

-Full replacement of PGP.
-Does not use any patented algorithms.
-GPLed, written from scratch.
-Can be used as a filter program.
-Full OpenPGP implementation (see RFC4880 at RFC Editor ).
-Better functionality than PGP and some security enhancements over PGP 2.
-Decrypts and verifies PGP 5, 6 and 7 messages.
-Supports ElGamal, DSA, RSA, AES, 3DES, Blowfish, Twofish, CAST5, MD5, SHA-1, RIPE-MD-160 and TIGER.
-Easy implementation of new algorithms using extension modules.
-The User ID is forced to be in a standard format.
-Supports key and signature expiration dates.
-English, Danish, Dutch, Esperanto, Estonian, French, German, Japanese, Italian, Polish, Portuguese (Brazilian), Portuguese (Portuguese), Russian, Spanish, Swedish and Turkish language support.
-Online help system.
-Optional anonymous message receivers.
-Integrated support for HKP keyservers.
-Clears signed patch files which can still be processed by patch. you can free download GnuPG for Mac 2.0.14 now.