Unless the AFCAD is wrong and needs other fixes, there's no need to edit the parking size in the AFCAD "just" to force GSX to accept an airplane in a parking the developer decided was too small for that airplane.
If that's your goal, and you don't have other reasons to fix the AFCAD ( for example, to accept bigger size AIs ), the proper way to do it is do edit the GSX Wingspan setting, which will control if GSX is allowing that airplane type there, and possibly the parking radius in GSX, which controls the initial positioning of ground vehicles ( although it's always best to place them individually ).
Gate 17 in the afcad has a set radius of 33.10m for A350, 777, 787 sized aircraft and that maxwingspan is 48.0 so there shouldnt be any issues should there???
That's normal. Anything you set in GSX always has precedence over the AFCAD setting, which is *another* reason why editing the AFCAD for GSX is useless, unless you need to edit it for other reasons. So, in that case, you can set the AFCAD size as high as you want, but the 48 Wingspan in GSX would prevent any airplane with a larger wingspan to get there. Since A350/777 have a wingspan of 65 meters, you must set it at least that value in GSX.
Which is why, GSX has that little airplane icon, so it can set the right wingspan for airplane you are currently in, so you might find it easier to use it that way.