It’s also possible to simply buy the mod you want for your Gatling Gun, as long as you know where to look. Keep in mind, however, that the stock at vendors in the game can rotate out from time to time ...

Fallout just isn't Fallout without mods. In fact, Bethesda Game Studios RPGs simply aren't the same without mods. In tandem with the creation kit and console commands, you can do anything in Fallout.