Could it be made mandatory with some of the addons i mentioned??
Sorry, but GSX is an FSX add-on, not an add-on for fs passenger, fs captain or airhauler, so it's not supposed to enhance them.
In order to have GSX "mandatory" for those add-on, if with "mandatory", meaning they wouldn't progress to their next phase or consider a flight to be completed if you haven't used GSX, it's THOSE add-ons that will have to be modified, how you would expect GSX could enter into THEIR code and make THEM behave differently depending if you used GSX or not ?
BTW, we are working to a GSX expansion product right now, that will generate Cargo missions with a purpose and THAT ONE, of course, will interact with GSX.