Gmax is a product of DISCREET and can be found at their site
www.discreet.com you will also find some tutorials as well here.
You will also need the gamepacks for the sim you plan on building for As there are seperate ones for each sim
FS2004 gamepack
FS2002 gamepack
And so forth
You will also need the SDK,S(software development kits) for the sim as well you will need the bgl compiller in the SCENERY SDK
Word of caution if you have no experience using 3d grafics program GMAX might be hard to learn how to use.
It is a great program it is just not the easiest to learn how to use for a beginner.