GSX doesn't refuel the aircraft, when this one has been flagged to use a custom fuel system, and the FS Labs is surely one of these cases. In addition to that, due to its integration with GSX, most of the GSX functions are controlled by the plane itself.
The first test you should do is: does refueling work with another airplane ? If yes, then the problem is not GSX.
That doesn't mean the problem is the airplane either.
In general, GSX simply cannot crash the sim, especially P3D V5, since the only thing that could theoretically crash P3D4, DirectX 11 access, is just not there in V5, which uses its own specific version of the Addon Manager .DLL that, for absolutely safety ( so we can't even call DirectX code by mistake ) has been compiled without any reference or dependence from Direct X.
The reason for this was because, due to the still not perfectly stable nature of P3D V5 and DirectX 12, we wanted to be absolutely sure nobody could suspect GSX as a possible cause of crashes. It could have crashed P3D V4, at least in theory, but of course it never did, but since not many are aware of the exact differences between DirectX version and we advertised a lot the fact we used DirectX 11 in P3D V4, we made a specific DirectX-free version of the software that it's used in P3D V5, so even the theoretical chance of a crash is just not there anymore in V5, surely not due to GSX.
Of course, again, considering how tricky DirectX 12 is, which usually result in the sim crashing when are close to VRAM exhaustion, it's possible that, if you already close to VRAM exhaustion before calling GSX, which is possible with a complex airplane on a detailed airport (and of course, depending on your available VRAM ), just adding the extra objects called by GSX might be enough to trigger a crash, which in P3D V4/DirectX 11 might have just caused a momentarily slow down.
If this is the reason, there isn't anything we can do, your only option is to lower your graphic settings to consume less VRAM.
The VRAM usage indication that can appear in the fps counter might give you some indication if you are in fact close to VRAM exhaustion when you call refueling.
If you have plenty of VRAM, then GSX is not even the indirect cause, so it's something else entirely, possibly in the airplane code, which is why it's always best to test with multiple airplanes.
