CSs have all the tech that have been researched by 2 major civs. So if two civs have scientific theory done but you don't, they still grant you their coal, even though you don't even see it yet. same with oil etc.
If you are the techleader and think of getting a resource through an allied CS Tesujis problem usually occurs. That was particularly annoying in vanilla (as you can't really know if the CS already know the tech/will grant the resource to you). Since G&Ks though you can pay them to improve that resource (and I believe that works as soon as you can see it).