I am thinking of making a lv 20 Gestalt module. The adventure is to sneak into Erebus the 3rd circle of hell and cook the books of Mammon. The war of hell runs on red tape seems fitting to strangle them with it.
Can anyone help me come up with obstacles the players should face?
The first challenge seems to be to sneak pass Barbatos in order to get pass the first layer.
I cannot see how you could reasonably write a challenge for an arbitrary group of gestalt level 20 PCs. For a start PCs could bypass the first layer entirely and plane shift to the 3rd, and that's just a 5th level spell.
That's without getting into the widely divergent capabilities possible for level 20 gestalt characters. Take the differences possible between midlevel characters and turn them up to 9000.
I don't think plane-shift allows you to pass between layers. Way too many outsiders can plane-shift at will.