Pd is a real-time environment for audio and MIDI. It was written by Miller Puckette, who created FTS/Max when he was at IRCAM. Basically, Pd can be seen as the next generation of real-time visual programming languages. GEM runs inside of the Pd environment.
You can get Pd at http://www.crca.ucsd.edu/~msp/software.html
Pd is sponsored by a grant from Intel (http://www.intel.com)