The Gatekeepers used the Dread Mines tunnels as secret meeting place after the 'Path of Unity' project was cancelled and after their cult got banned in the whole continent. They used to meet there because it's the place in which Keliask the giant and his followers were put to rest in peace.

An underground cult from a bygone age. Prayed for the coming of a prophesied messiah, whom they called Toroah. Banished from the civilized world by the religious authorities of their time, both for heresy and for their dangerous, forbidden prayers.
Toroah: The messianic figure of the Gatekeepers cult's mythology. Said to wield a weapon called 'Vandal Heart' and to embody 'serenity and fury'
Keliask's Tomb: the resting place of Keliask, a legendary giant, and his equally-large followers. The Gatekeeper cult revered his kind as prophets of Toroah, and the giants protected them in exchange for their tribute. When the Gatekeepers were forced underground, the giants died of starvation, and were laid to rest here.
-Vandal Hearts: Flames of Judgment ingame encyclopedia
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