The ACU Gunner is a member of the Asset Containment Unit. When Simon Masrani takes it upon himself to personally go after Indominus rex, he has a BFG (an M134 Minigun) put on his private helicopter, and this guy mans it. Unfortunately the guy's aim sucks and all he does is drive the I-rex towards Jurassic World's aviary, where it smashes through the glass wall, unleashing a horde of flying reptiles, which of course fly straight up and attack the copter.
As amateur pilot Masrani struggles to keep the craft steady, the gunner falls out the open side door. There's a glimmer of hope as he manages to grab the gun and save himself, but that glimmer is quickly snatched away, along with him, by a swooping Pteranodon which carries the unfortunate trooper away kicking and screaming.