GCD 6 exists in a few different flavors. The GCD 6 ArcGIS Add-In is a C# project coded in Visual Studio that makes calls to an underlying GCDCore library written in C++, as well as a RasterMan Library also written in C++. There is a command-line version of GCD that makes calls to the same C++ libraries. Given how finicky the Add-In program is, we have not made that open source and instead freeware. However, the underlying libraries that all the computations and GCD functionality is based on are all open source.
The GCDCore Library will be posted to a GitHub repository shortly.
The RasterMan Library is used in GCD and other tools and is an open source library of common raster geoprocessing calls written in C++ and available in Win32, Win64, Ubuntu12.04 and OSX 10.10 builds. A GitHub repository of the source code can be found here.