I'm having the same issue.
I had to flee when I initially took on the last group of Vints because I didn't expect the mage to be level 16, so half my group went down. Is it possible that Gatt went down in the fight and just didn't reappear? If so, is there a console command that can "resurrect" him and fix the bug?
I've done the quest before, so I get the gist of Gatt's side of the conversation, but its still sort of frustrating. I tried running back to the start of the quest, down by the tents, tried reloading my last save, restarting my computer, still nothing. I'm told that a few have tried reinstalling the game to fix it with no success, but it's not worth the trouble for me to try it.
