The Gainer is a unique .44 magnum modified to fire incendiary bullets. It does so by using the Incendiary Legendary effect, which adds 15 points of burning damage over time. This magnum also comes equipped with the standard receiver, bull barrel, comfort grip, and a reflex sight.
Can be found next to a skeleton in the Vitale pumphouse, an unmarked location south of the Hub City Auto Wreckers, and east of Finch farm, behind a code-locked door. Looking at the wall adjacent to the code entering device, the code can be seen written upon it (0, 4, 5, 1). Being as the player character does not have the option to input a 0 in the beginning of the code, a 10 must be inputted instead, leaving the actual code as 10, 4, 5, 1.
Behind the scenes
The 0451 code used on the door that The Gainer is locked behind is a reference to Deus Ex, where it's the code used to open the first door in the game, as well as other games by Looking Glass and Ion Storm such as Thief and System Shock where it also appeared. Originally it was the key code to get into Looking Glass Studios offices in Cambridge.
Lead level designer Joel Burgess described The Gainer as a "quiet high-five" to dev friend Steve Gaynor on Twitter because of his "love for a nasty-ass video game magnum."