Gnu Privacy Assistant

The GNU Privacy Assistant (GPA) is a graphical user interface for the GnuPG.GPA utilizes GTK (the GIMP Tool Kit) and compiles for various platforms.
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.