I have now chosen the airport that I will use as a test field for developing the gPoly tool. It is Marshfield Municipal Airport (KGHG) in Massachusetts. So I have drawn some of the aprons and taxiways and am testing now if I can export them correctly to FS.
At first I was exporting with the FS2002 style code used for ground polygons most of the time. As long as I export everything with one reference point things looked fine, but when I divided the polygons over different reference points, to have better control over the performance, I noticed that there are gaps visible between the polygons. In FS2004 the same approach worked fine, so it seems the gaps are caused by the curved earth in FSX. At the moment there is a very useful discussion going on at the FSDeveloper forum about this issue. I still hope that once I understand how FSX corrects the polygons for the curve of the earth, that I can export them again with a compensation so that they line up correctly.
To learn more about this curved earth, I decided to start with exporting the ground polygons as FSX MDL files as well. In these MDL files I am bending the ground polygons so that they follow the curve of the earth. This approach seems to work quite good, although I still have some issues with gPoly not always triangulating the polygons correctly. Between the different MDL files I still see a small gap sometimes, but it is much smaller than in the FS2002 style approach. I hope that by improving my math a bit I can get things aligned correctly again.
The fact that the FSX MDL files seems to work quite well is quite exciting, that would open a lot of possibilities to make better ground polygons. For example with reflection when it is raining or with bump maps. I am looking forward to continue the experimenting.