well first you need a plug in for gmax which is called nwmax, this can be found here
http://nwmax.dladventures.com/), you use this to export your model as a .mdl file, once you have done that you need to use mdlops's read and write function to convert the ascii .mdl you extracted from gmax to the final .mdl which the KotOR game can read, mdlops has tutorials that come with the program that helps allot.