The Milk Gremlin, spawned from either Lenny, George, Daffy or one of their other spawnings, was just a regular gremlin until it drank a hormone that has a liquid genetically-related to milk. Due to getting shot by an Uzi machine gun, the substance leaked out of small holes in his chest. Two gremlins laugh about it, and when the Gremlin who drank the hormone sees it, he also laughs with them.
Later, he was in the lobby with most of the gremlins (except Greta) and was doused with water before getting killed along with them by the Electric Gremlin who was unleashed out over most of them.
As demonstrated by Stripe and the Brain Gremlin, a Gremlin can get killed by a bullet, such as the Poker-Cheating Gremlin and the Beanie Gremlin. However, Mohawk shoots many bullets at the gremlin that was drinking milk and only small holes come out of its' chest, unless the potion the gremlin was drinking made him immune to bullets... all kinds of them.