RTF is a global modification that replaces all towns in the game.

You've created some glorious bastards! But as I recall, in your mod, the "demon" ability, which is a spawn of darkness, means that a chaos dwarf with a pendant can open gates with these creatures, right? Or can you specify in the game files which creatures can be summoned through the gates?

A separate line in the abilities determines whether gates can be opened, so they can't be opened... but summoning cave lords will work; I configured it for the titans so that they can summon them from the corpses of enemies.

Added 1 minute ago
Oh, lol, I remembered about opening gates in another way. In the old mod, I made the Academy units, which were replaced with elementals, open gates by assigning Inferno units to their basic creature list. It was some kind of hellish shamanism.