Pardon the question if it’s obvious....but what is GSX?
It's our most popular product, which adds several ground services. The latest version provides jetways, and KLAS (and many of our sceneries too) requires it for jetways to show.
It's entirely free to use, with no limitations, at all our airports so, you can consider it as a mandatory download for almost all our products.
It must be installed only once, then it's kept updated by our Live Updater so, in the end, even if you must download 1.0GB now, you do it only once, and this allowed the installers for all our sceneries to be smaller compared to other similarly sized airports, since we don't duplicate all models and textures that come with GSX.