Making a GC takes a bit of work. In a nutshell (all the following assumes using FoC):
-make actual entries for the GC in Campaigns_Underworld_GC.xml (you'll need a GC entry for playable faction in the GC; for example, a standard FoC GC has three entries, one Rebel, one Empire, one Consortium). Make sure that what you put in the Campaign_Set line is unique (unique to the campaign. It should be the same for all three faction entries for the GC but different from any other GC). For the story_name lines, make sure that whatever number you pick (use their format) is unique to that GC as well. Otherwise, refer to existing GCs, it's pretty straightforward.
-Make the Story_Plots_Sandbox and Story_Sandbox XMLs for your GC. Again, the number you put in the name should be unique to that GC. Otherwise, making these files should be *extremely* straightforward: most of FoC's different GCs' Story_Plots_Sandbox and Story_Sandbox XMLs change very little between GCs.
That should at least get you started. Hope this helps!