XML files:ĬAMPAIGNS_UNDERWORLD_GC.XML (The GCs themselves) Assuming you are working with FoC, you'll need to at least edit the following. There's no tool for it, it's all through code. What exactly are you wanting to change/modify? There is a lot involved depending on what you're doing so I don't want to spam a bunch of info at you that you may not even need. I dunno if there's a tutorial anywhere for this one.
