GTX went around the TOS and started offering “VPS’s with FiveM FXServer preinstalled” which is still violating the TOS. Even offering to install FXServer is a grey-zone because they still offer FiveM.
That was an Element (Member of the CitizenFX Collective) aka, FiveM Developer.
I doubt GTX will be unblacklisted, due to their sneaky practices trying to get around the FiveM’s TOS. It’s honestly not hard to get a VPS then install FXServer on said VPS.
Also, the reason why your key isn’t working is because you’re using GTX Gaming, which again, is a blacklisted GSP.