So what do I want my GURPS Cabal to be about?
Well, let's imagine what a typical GURPS Cabal game might be about. First, something happens. Perhaps a creepy mummy slips out of its coffin at a museum and kills a dude and steals some artifacts, or perhaps someone inherits a creepy reality-shard journal that speaks of things that never happened but will, or perhaps your passer dies. Something happens, and being an occult game, you need to investigate what happened.
We already have a game that makes heavy use of investigation: GURPS Monster Hunters. Of course, the premise of GURPS Monster Hunters is that you're out to kill the monster while in GURPS Cabal you are the monster... but on the other hand, is it really so different? What's the difference between a group of Black Ops soldiers hunting down that mummy to put a stop to his rampage, and a Cabalist hunting down that mummy to ask it a few questions and/or gain access to its dark secrets? Or perhaps to put a stop to its rampage because it's growing inconvenient. So we simply rip those rules wholesale from Monster Hunters.
So where do we get the rules for that? Well, conspiracy comes in three, distinct elements. The first is about understanding power structures, their interconnectedness, and how a conspiracy needs to play out. Our go-to resource for that would be GURPS Illuminati, but that doesn't help us too much. Mostly it just offers GM advice and setting suggestions. Conspiracy is a mode and it discusses that for us. Otherwise, Action offers some suggestions regarding convincing organizations to do things for you ("Pulling Rank"), and we just toss in some rules about Administration and Politics.
To do this, we turn to GURPS Social Engineering, which discusses dealing with organizations (surprisingly irrelevant, though could be useful for running a lodge), manipulating people (alright), recruiting people (there we go), blackmailing them (bingo!) and brainwashing (Do tell!). When I was finished researching this bit, I think that was the first time I really began to be afraid of the cabal.
Finally, we need to hide evidence of the supernatural. Deception. Remember how that mummy murdered someone? What if it got caught on camera? What if the police begin to investigate the murder? This could expose more of Cabalistic influence than we would like, thus we need to dispose of the body, alter the camera evidence and send the police off in some other direction. Monster Hunters has a sparse discussion of this, to my surprise, but Action 2: Exploits, always my hero, rides to my rescue. It has an entire section on "cleaning," hiding evidence, faking evidence, making up cover stories, disposing of bodies and etc. I could probably go into more detail, but this will do.
Now that we've hidden our mummy from the world and pulled the black ops team off the trail, we can walk right up to the mummy, help him with whatever he's doing, and get him to join the cabal. Easy. As long as everything goes smoothly, we'll have a new ally to the cabal and no muss or fuss. But when does anything ever go that smoothly? No, at least one black ops soldier is still on our trail, the mummy isn't interested in cooperating, and somehow the mafia got involved. What would a good game be without some pulse-pounding action?
The mode of our action will vary. We could treat this game as full-on horror, but I think if players are the monsters, they'll want something pulpier. We could go full-on action heroics, but that strikes me as overkill. Do we really imagine Cabalist assassins as charging a room full of enemy soldiers while firing round-after-slow-motion-round into their midst and back-flipping out of the way of bullets? I don't. They seem grimmer than that: Snipers, thieves, knives in the dark. If it comes to a fair fight, it's bloody and terrifying, and the only things that can do ridiculously inhuman things are inherently inhuman: Super-fast vampires, super-strong werewolves, and so on.
I didn't turn to anything for this, though both Monster Hunters and Action have some good rules on combat. I'll use more of Horror's rules, though, and I've been running combat-scenarios for awhile now, so this requires little investigation.
But those three parts together put together the core of how a game will play out, leaving only one last thing to worry about...