I was asked if I wished to activate GSX? I've tried both YES and NO responses
You mean you got the "Do you want to activate Fsdreamteam GSX on this PC ?" If yes, it's entirely normal the question will be made after you replied NO, because the only reason why you get that question, is that because you DO have a Serial Number for GSX in the Windows registry, coming from a previous installation.
So, you should always reply YES. What happens next ?
A reason why the activation doesn't "stay", is if you are running the sim in "Compatibility" mode, like XP under Windows 7 or Windows 7 under Windows 10, etc, which is not the correct way of operation and not how the sim is installed by default.