Download the Glest editor pack, unzip it, look for the plugin for 3DS Max. There are several, pick the one best for you (I think 5 works with 4, so if you have 4, use the 5 plugin) then put the plugin into your 3DS Max>Plugins Folder. Load up 3DS Max, load up the model, select export, select the G3D format and there you go.