Gmax is a freeware version of 3ds Max with a limited feature set intended for game content creation: modeling, texturing, animating, and basic materials. The MAXScript plugins Chimp and Bluestreak (compact versions of scripts) can be used to export JMS files from Gmax.
Halo modders looking for free 3D modeling software should use Blender and the variety of add-ons that now exist for it.
It is becoming increasingly difficult to find reliable download sources of Gmax online. It was officially discontinued by Autodesk in 2005 and TurboSquid stopped hosting it in 2021. Gmax was also used for custom content creation in Trainz and Flight Simulator 2004 so you may be able to find mirrors hosted by those communities.