The Fortress is a kind of a garage, surrounded by a moat of glowing green goo. This intimidating location is the home of the Gun Runners, and contains their manufacturing facilities, where they create high quality weapons that pack a definitive punch.
With a high enough Barter skill you can buy and sell back to the Gun Runners for more than the original cost, and since the amount of money they have resets after every transaction, this can be a source of unlimited cash.
Their stock also resets after every transaction, so be careful on what you are selling or save before you buy/sell there.
It is possible to simply run through the moat in the northeastern corner, bypassing the guard at the bridge.
Surprisingly, it was never intended for the player character to talk to Gabriel, resulting in the conversations being like normal as if having a regular Intelligence above 3.