The gateway was destroyed by disbelievers among the Geisterdamen, most notably the Loremistress Milvistle. It seems very likely, , that this action took place just before the attack on the Castle and may have been related to the massive destruction.
In the aftermath of the damage, the High Priestess Vrin was able to reconstruct the Summoning Engine with Prince Aaronev's help, but was the gateway to return to her homeland. Quite obviously, the techniques for building shk-mah generators (Hive engines) was also retained in some way.
This state of exile seems to be coming to a close, as the Geisterdamen have an avatar of their goddess with them now who can rebuild the gateway from their new base, wherever the tunnels of the Sturmhalten Deepdown might have taken them.