I believe there have been problems with ORBX and GSX in the past due to the way ORBX creates their scenery. The target users of ORBX airports (mostly small GA) and FSDT (Commercial and large GA) are two entirely different groups for the most part, with only a small fraction of ORBX products catering somewhat to commercial simmers (Monterey, Melbourne, etc.), so therefore, their design techniques are a little different, and one thing I've noticed is that their afcads and GSX don't like each other, Juneau comes to mind here. I don't have any ORBX airports because the only flying I do is for my VA and there are no airports my VA flys to in ORBX's lineup, so keep in mind I don't have personal experience in this matter. This comes from browsing the forums and seeing many posts about these sorts of compatibility issues. If I were you, open up the AFCAD and check if there are any errors in the AFCAD (missing nodes, missing connections, etc) that could mess up GSX. If you can't find anything, decide if you want the curved runway or GSX more at Sumburgh, and, if you want GSX more, reinstall the scenery using the flat runway option.
