Layer Cake

An unnamed man - appearing in the end credits only as XXXX - is a London underworld figure who has established himself in the cocaine business as one of the city's biggest suppliers, whilst presenting himself as a figure with legal business operations. Considering himself a businessman and a patient person, he leaves the darker aspects of the business to his henchman Morty, who had previously spent 10 years in prison for murder. Once he has earned enough to ensure his long-term financial security, XXXX plans to retire.
Approaching this point, his retirement plans go wrong when his powerful supplier Jimmy Price instructs XXXX to track down Charlie, the teenaged daughter of Price's associate, Eddie Temple, who has fled a rehabilitation centre along with her drug-addicted boyfriend. To track her down, XXXX enlists the help of two con men known as Cody and Tiptoes. To the frustration of XXXX, the boyfriend is found dead and Charlie has apparently been kidnapped, although no ransom has been demanded.
At the same time, Price instructs XXXX to organise the purchase and distribution of one million ecstasy tablets from a low-level gangster styling himself as "The Duke". XXXX does not know the pills had been stolen from a gang of Serbian war criminals based in the Netherlands, and who have been in hiding since the Yugoslav Wars and are now large-scale drug producers.
Meanwhile, XXXX encounters the Duke's nephew Sidney at a bar and becomes attracted to his disillusioned girlfriend Tammy. XXXX tries to broker the sale of the ecstasy pills to Liverpool gangsters Trevor and Shanks, but they refuse, informing him of the drugs' purloined nature and of the fact that the Serbs have sent an assassin, known as Dragan, to find the pills and kill those responsible for their theft. Because the Duke had mentioned XXXX's name to the Serbs beforehand, he, too, is now in danger. Members of the Duke's gang soon start turning up dead and the Duke himself goes missing.
XXXX arranges a tryst with Tammy, but before it can be consummated, he is kidnapped by henchmen of Eddie Temple (the father of Charlie). Temple informs him that Jimmy had lost Â£13 million in a failed investment in Africa and is now hoping to sell the pills to recoup his losses. He demands instead that XXXX sell the pills to him, and plays him a recording indicating that Jimmy has been a long-term informant for Scotland Yard and intends to betray XXXX to the police as soon as the deal is done, planning to gain clemency from the deal and also take away XXXX's wealth.
A furious XXXX shoots and kills Jimmy at his home, but does so with a gun that Gene, Jimmy's henchman, had used to commit a prior murder, putting both himself and Gene in danger. XXXX then discovers that his accountant, to whom Jimmy had referred him, has embezzled all of his money and disappeared. Gene and Morty, after being given evidence of Jimmy's snitching, acknowledge XXXX as the new acting boss and show him the corpse of the Duke, whom Gene had killed along with Duke's girlfriend, Slasher, when the latter threatened to alert the police about the pills.
To recoup something from the fiasco, XXXX tips the police off about the Duke's hideout, who then conduct a raid. XXXX is intentionally present during the raid to feign purchasing the pills from the Duke's crew, and Dragan, who has been following XXXX, watches the raid from afar as the police confiscate the drugs and as XXXX and Duke's crew escape. Meanwhile, XXXX has arranged for Cody and Tiptoes to con the raiding police unit out of the pills. XXXX delivers the severed head of the Duke to Dragan as a peace offering, who reports back to his Serbian boss that the police have seized the drugs. Cody and Tiptoes return the drugs to XXXX in exchange for a fee. However, when he attempts to sell them to Eddie Temple at the latter's bonded warehouse, Temple's henchmen rob his crew of the drugs at gunpoint. To end XXXX's faux assignment given by Jimmy, Temple reveals that he has put Charlie back into rehab.
To satisfy him, Temple then gives XXXX a "welcome to the layer cake" in the form of a membership to the Stoke Park Country Club of which he is a member and Jimmy was. Expecting betrayal, XXXX successfully arranges for Trevor, Shanks, and young Dizzy to steal the drugs back from Temple's men. The gang assembles for lunch at the Stoke Park Club, honoring XXXX their new boss. However, he demurs, stating his intention to stick to his planned retirement. When leaving the club with Tammy, mentally running through the names of his fellow murdered businessmen, he is shot by a heart-broken Sidney. XXXX collapses bleeding, and the film ends with him twitching on the club's front steps and thinking of his possible imminent death.

Question: Whose gun did XXXX use to kill Jimmy?
Answer: Gene's.