The Milk Gremlin, that spawned from either Lenny, George, Daffy or one of their other spawnings, was just a regular gremlin until he drank a hormone that has a liquid genetically-related to milk. Due to getting shot by an Uzi machine gun, it leaked out of small holes in his chest. Then two gremlins laughed about it, and when the Gremlin who drank the hormone saw it, he also laughs with them.
Later, he was in the lobby with most of the gremlins (except Greta), and then he 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 shot many bullets at the gremlin that was drinking milk, and only small holes come out of his chest, unless the potion the gremlin was drinking made him immune to bullets, and all kinds of them.