There is not a lot of info on the Grey Ghosts so I think you can run it however you think would suit your game best. There is no wrong answer.Personally, I think I would allow the PCs the long rest, but maybe have the Grey Ghosts launch an attack on them later either in the tunnels or Gracklstugh proper (a group of surface worlders in the grey dwarf city likely sticks out just a bit). The Ghosts are potentially an organization that could continue to harry the party as long as they remain in Gracklstugh.
My biggest gripe about OotA is that there is not enough content to get the party to the intended levels, so I have been taking any excuse to add in my own material. I would use an escaped foe as a chance to expand the Grey Ghosts as antagonists, maybe add another Ghost base for the party to take on. For example, if the party has some of the NPCs from Ch. 1 accompanying them, maybe have the Ghosts kidnap an NPC so the party has to go after the Ghosts to rescue them.
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