Making ground polygons with GMax always seems to be a hot issue, especially the fact that you have to use the FS2002 gamepack and apply some tweaks to make things work. So this challenging part of scenery design gets discussed a lot on the forums. But now there is a new tutorial that might help you. In it Bill Womack explains how he made the runway for his Plum Island scenery. So have a look at this nice tutorial.