I don't know what do you mean when you said "I already lost old GSX due I guess to Microsoft updates".
The "old GSX" ran on FSX or P3D, MSFS came 13 years after FSX, it's not as if you can call it an "update". In fact, it's quite the opposite: because we released a new version for MSFS as well, you can get it back, that is the opposite of "losing it", something you could say if you switched to MSFS before GSX Pro came out.
I'm sorry if the 11 years of free updates ( which are still continue to come, at least for the main code ) we offered in FSX weren't enough for you, but there must be a limit after which most users would consider reasonable to ask to pay for a new version. Most software I use right now, comes with yearly update fees...
About releasing it in the MS Store, is not currently possible, because Microsoft doesn't accept products that requires an external .EXE to run. That is exactly the same reason why you won't have things like the Fenix A320 or the Leonardo MD there.
