First, it's important to realize that GSX Pro works on PC only so, if the only copy of MSFS you have now is on Xbox, there's no way to use GSX Pro and there never will, since a product like GSX, which requires an external .EXE application to run, cannot technically run on Xbox, ever.
Once this is clear, if you buy a GSX Pro license, it will work regardless of the version of MSFS ( for PC ) you have installed. In fact, the installer even supports installing in both versions at the same time. The GSX activation key is completely separated by which version of MSFS you use, as long is on PC.
