Same problem here.
Since yesterday (12/15), coinciding with installing the latest Windows security updates (KB4533002, KB4484193 and KB4530684), I have the very same issue. GSX is not appearing in the in-sim menu, when checking the AddOn manager from within the sim it says both GSX and GSX Level 2 are in Trial Mode. Running the FSDT Updater (excluded from Windows Defender, Windows Defender disabled altogether) results in updating various MS Visual Universal Runtime instances over and over again with every single run of the updater, but to no avail as far as changing anything with regards to the functioning of GSX in-sim.
I also tried dagauy's suggestion - to no avail.
I'm wondering if it would be worthwhile checking a possible interference with the latest Windows updates as per above?
Thanks,
Jan