She was played by Leah Gibson.
Gemma Wheeler was a bartender who approached by Monica Bolland (the film's protagonist), regarding a job at her bar. The job was part of Monica's cover, as she was investigating white collar criminal Collin Murphy regarding the murder of her FBI agent husband, Josh. While forming a friendship with Gemma, however, Monica was taken aback by the revelation that Gemma slept with Josh during his undercover work.
During the illicit relationship, though, Gemma had sensed that Josh was cheating on her, and while spying on him, she spotted him leaving with Monica. Furious, Gemma confronted Josh and held him at gunpoint, with Josh revealing that he's an FBI agent. She later shot and killed Josh, and took his watch and $500,000 in cash from his place. After Murphy's arrest, Monica revealed her true identity to Gemma, who said that she had proof that Murphy had Josh killed. As Gemma left, however, Monica spotted Josh's watch in her place, which gave Gemma away as the killer.
Back at the bar, Gemma showed Monica photos of Josh at the motel where he left Monica, and then boldly confessed to killing him. The villainess later pointed her gun at Monica with intent on killing her, while stating that Monica was "the best friend she had in a long while." Gemma's gun was revealed to be unloaded, as Monica later showed her the bullets and stated that she knew that Gemma was lying to her. Gemma later attacked Monica, starting a brawl between the two which saw Gemma attempt to kill Monica with a broken bottle. At that moment, Aiden Savage arrived and shot Gemma to death, saving Monica.