A gimmick is a term used to describe a niche style of gameplay or level design that may only be favorable to a subset of Doom players. Maps such as Doom II's Barrels o' Fun, feature a gimmick where the purpose of the level revolves mostly around the player outrunning a chain of exploding barrels to get to various safe zones. Another example is MAP08: Tricks and Traps, which features various gimmicks that do not necessarily pay respects to the usual Doom gameplay a player may come to expect. Some levels and megawads are designed around gameplay gimmicks such as Cyberdreams and Archvile Jump; while others revolve around a gimmick in level designing, such as 10 sectors, Claustrophobia 1024, and 1 Monster Megawad.
Secret levels are also often gimmicky: Fortress of Mystery relies on exploiting infighting, Warrens make the player believe he is replaying through the first level again before transforming the entire map, Wolfenstein and Grosse are recreations of maps from Wolfenstein 3D, Go 2 It turns Entryway into a slaughterfest...