I just tried timing my startup, and I've got the following results, from clicking the icon, to when the Main Menu appears:
- Empty Community folder - 2.30
- Only GSX linked - 3:13
- Many addons, no GSX - 4:10
- Many addons and GSX - 4:54
So, it seems that, at least on my system, GSX adds about 43-44 seconds to the overall load time. However, it's difficult to say if, over a certain amount of add-ons ( I don't have that many, but I do have some that adds lots of objects, like FSLTL ), the time taken by GSX might start to increase, when in my case it was linear.
In any case, what you are asking is not possible, all GSX objects are pretty much interdependent from each other, in order to be as efficient as possible with the textures size, so they have very complex dependencies, to be sure textures are not wasted, so they are reused on many objects as much as possible, meaning if you remove something you might not think it's important, it might have a massive impact on the things you need.
Also, even if you only use Pushback and Marhallers, GSX will still have to prepare the gate with all required objects, so it will cause errors if any of them are missing.