GMC is a Monte Carlo program used to simulate the results of e906. It includes a dimuon generator based off of FastMC and also a background generator. After the particles are generated, Geant4 is used to track them through the spectrometer. The best way to install GMC is through the SeaQuest software distribution due to the large number of dependencies that are automatically downloaded and setup for you. Directions are in document 1066 on the doc-db, starting on page 8.