She was quickly established as being an arrogant and judgemental women, as shown by her (along with her friends Stacy and Vicky) condescending to Amy Mitchell (the film's main protagonist) regarding her status as a working mother. Ironicially, though, Gwendolyn and her friends also expressed disdain for Kiki, an overworked stay-at-home mother. Gwendolyn was also shown having a crush on Jessie Harkness, a widowed father at the school.
At an emergency PTA meeting, Gwendolyn revealed that she was organizing a "police force" for the school's upcoming bake sale to monitor for treats in violation of her extensive rules, and tried to have Amy volunteer as punishment for being late to the meeting. But Amy, fed up after the stressful day she'd had, vented to Gwendolyn about her struggles before deciding to quit the PTA. Amy later on told Gwendolyn she wouldn't be going to the next PTA meeting about the upcoming PTA elections, angering Gwendolyn as she proclaimed that Amy was playing a dangerous game. Gwendolyn eventually snapped when Amy brought store-bought donut holes to the fundraiser, knocking them to the ground and telling Amy she was going to destroy her life, while also throwing insults at both her and her children, knocking down a woman carrying cupcakes as she stormed away.
Gwendolyn makes good on her promise to Amy by having her daughter Jane benched on her soccer team, threatening to have the soccer coach fired if he didn't comply with her demand. When Amy learned what happened, she angrily confronted Gwendolyn, with the arrogant villainess telling Amy her actions were permitted due to soccer being a PTA-sponsored activity. Upon learning that, however, Amy informed Gwendolyn that she was going to run against her for PTA president, to which Gwendolyn mocked Amy while making snide comments about how Amy had caught her husband Mike cheating on her. When Gwendolyn saw Amy was holding a party to announce her PTA campaign, she held her own party with Martha Stewart providing catering, only for her plan to foil Amy's campaign failing when several of her party attendees went to Amy's when she began boring them with her self-aggrandizing lecture on how she planned to impose year-long schooling at William McKinley.
After going to Amy's party and seeing the immense support she'd gotten, the evil Gwendolyn plotted to ruin her chances by planting marijuana in Jane's locker, resulting in her being kicked out of all her extracurricular activities and turning her against Amy. Gwendolyn then began spreading the news to other mothers in the school, using it to paint Amy as a reckless and irresponsible mother who shouldn't be PTA president. Gwendolyn's plan also resulted in Amy becoming despondent and initially planning to skip out on her candidate speech, with Gwendolyn gleefully going first and informing Principal Daryl Burr that she would talk for however long she wanted. But after Gwendolyn finished her speech, Amy (having been inspired by her friends Kiki and Carla Dunkler to stand up to Gwendolyn) arrives and makes a speech that wins back her peers' favor.
While Gwendolyn was initially dismissive of Amy's speech, she later found her rival crying in her car, with Amy going to comfort her. While originally assuring Amy she was fine, Gwendolyn proceeded to reveal that her life was not as perfect as she pretended it was, as well as how her controlling and judgemental behavior regarding the PTA was her way of dealing with her frustrations. Gwendolyn also sincerely apologized to Amy for what she'd done to Jane and promised to make sure her name was cleared, at which point she and Amy reconciled and the film's ending showed her taking Amy, Kiki, and Carla out for a day of fun in her husband's private jet.