You definitely may not post GC on Curse! That would be against the license.
What is permitted is: if you have created a full modpack including configs and maybe patches and Minetweaker stuff and other files which are custom for your modpack. In that case, we give permission to include GC in your modpack's own download on Curse. The Space Astronomy modpack is an example.
Obviously, we don't give permission for you to do that with a "Galacticraft only" modpack. It would need to be a proper modpack.