Trivia: Jonathan Hyde plays both Mr. Parrish and the villain Van Pelt - a nod to Peter Pan, in which the same actor usually plays both Mr. Darling and Captain Hook. Interestingly, Robin Williams had played Peter Pan in Hook a few years earlier.
Trivia: Director Joe Johnston almost didn't cast Robin Williams because he was worried about Williams' knack for constantly improvising. However, Williams convinced Johnston that he understood that the film's tight pacing and special effects meant he couldn't improvise as much as normal, and gave Johnston the confidence to hire him. Johnston ended up letting Williams improvise fairly often, but only after filming scenes as written.
Answer: The game is essentially "resetting reality" back to the point Alan was first sucked into the game. So it is undoing everything that has been done - including taking away the gun and bullets Van Pelt had purchased.