This quest is optional.
After your Banishment from the Old Camp, you'll find Milten in front of the Old Camp, near the bridge. If you delivered the message to the Fire Mages, you'll remember him from the entrance; otherwise, you'll first meet him during the retrieval of the stone ring Focus Stone, fighting a Blood Hound. Either way, he'll tell you that Diego is watching the south gate of the Old Camp and that you should go see him. You can ask Milten a bit about the Old Camp's upheaval.
Diego will tell you about the Old Mine collapsing and Gomez's impending assault of the Free Mine. Moreover, he'll also tell you that he wants to schedule a meeting between himself, Milten, Gorn, and Lester in their usual place, and asks you to spread the word onto them if you find them. Only Diego will give you the quest; you don't have to talk to Milten first before talking to Diego.
Milten is, of course, watching the northern gate of the Old Camp.
Lester is still back at the fortress where you fought the Harpies to retrieve the Focus Stone, in the library room.
Gorn will be found, at that time, in Satura's room amongst the Water Mages. Remember that Saturas gave you a teleportation rune to offer as a piece token? You can use it yourself to get right back there instantly.
If you wait too long, the quest will end on its own, uncompleted. If you plan to finish the quest, you should probably do that as soon as possible; at the least, do it before you and Gorn go off on your next adventure together - freeing the Free Mine.