GMAT makes official releases to the public approximately once every 6-12 months.
The latest public release of GMAT is located at SourceForge Page. Note that if you wish you use the MATLAB interface, some additional configuration is necessary. See the MATLAB Interface documentation for details.
If you use Mac or Linux, or if you just want the latest code, you can choose to build GMAT yourself. Our public code is hosted in a Git repository on SourceForge:
See Development for compiling instructions.