Please make a list of the popular add-ons that have a 3d cabin which are not supported by GSX, and we'll have a look at them.
Not sure which A350 you are referring to but, if you are referring to the upcoming one from iniBuilds, why you assuming it won't be supported, considering we already support the A300 ?
We can't support the A310, because it was made for Microsoft and the external model file we need to patch is encrypted, like all Marketplace airplanes (even if they are free), so there's nothing we can do about this issue. Marketplace airplanes will never be supported, unless the eventual appearance of an "Attach API", which is available in Prepar3D, but not in MSFS.