Mindhunters

The titular Mindhunters are a group of young FBI students who are undergoing training as profilers. They are tasked by their instructor Jake Harris to travel to a small island off the coast of Virginia. This island is used as a training facility by the FBI and the military, and a mock town has been constructed there. Harris has arranged an elaborate training scenario for his students; they are to create a profile of a serial killer who has committed a murder there.The students include Bobby (a young man with a talent for fixing things), Vince (a wheelchair-using ex-cop who goes nowhere without his gun), Nicole (a smoker who is attempting to quit), Sara (a talented but insecure profiler who is also petrified of drowning), Gabe (an outside observer), Rafe (a very intelligent, caffeine-powered British investigator), Lucas (a supposedly fearless man whose parents were killed when he was a child), and J.D. (their leader).Once they arrive at the island and settle in, they begin their investigation the following day. The group encounters an elaborate, Rube Goldberg or Heath Robinson style trap; J.D.'s position of leadership prompts him to investigate it close-up, and the trap kills him via liquid nitrogen. Convinced that this wasn't an accident, the group heads to the dock to leave the island. Another trap is triggered, this one destroying the boat docked there. Tensions run high among the group, and they find messages from the killer that indicate when more victims will be killed.Several more of the profilers are killed, in ways that correspond to their personalities: Rafe through his need for coffee (drugged; decapitated and drained of blood while unconscious); Bobby through his desire to repair things (harpooned by a device he triggered while trying to shut off a water valve); Nicole through her need for nicotine (after finding cigarettes in a vending machine, they turn out to be laced with a strong acid); and Vince through his need to have his gun handy (it had been rigged while he was unconscious to explosively misfire).In between these events, the investigators use the resources at hand (including a fully-stocked forensics lab and computer database) to process the clues they have (including blood samples and an elaborate cypher, written in Rafe's blood). The results point to Sara, who denies being the killer; Lucas supports her, and suspicions seem to point to Gabe because of a map of the island found next to his bed. The remaining profilers elect to stick together, to keep an eye on each other. Later, Sara, Gabe and Lucas find Harris (who did not leave the island, though he led the profilers to believe that he had) and two other FBI agents next to him, all dead; Harris has been strung up to wires from the ceiling as a sort of marionette, just like the fake crime scene that they were to investigate. In the shootout that ensues, Gabe seemingly takes out Lucas. Gabe and Sara confront each other, each believing the other to be the serial killer. Lucas, who was wearing a bullet-proof vest, returns and jumps Gabe from behind. The two violently attack one another, eventually falling through a glass ceiling. Gabe starts to get up but is knocked unconscious by Sara, who then begins to tend to Lucas.Sara explains to Lucas the trick up her sleeve; she set a trap taking into account the killers obsession with time. However, this reveals that Lucas, not Gabe, is the mastermind. Lucas says he was the one who murdered his parents, but after no one suspected him, he grew bored and required more intelligent peers as worthy prey.Lucas muses that Sara has another weakness besides a fear of water: Me. Lucas tries to drown Sara but she kicks him into the water and an underwater gunfight ensues. In the end, Sara gains the upper hand. Lucas begins to taunt her about the evidence he planted until Gabe reappears. Lucas attempts to use the distraction to his advantage, forcing Sara to kill him.The film ends with Sara and Gabe flagging down a helicopter together. As they move to board, Sara wonders whether or not she has achieved profiler status.

Question: Who was Gabe?
Answer: An outside observer.