Gomez was seen laughing and skipping while working on the film -- which also stars Austin Butler, Bill Murray, Adam Driver, Chloe Sevigny, Rosie Perez and Daniel Craig -- on its upstate New York set.
Butler and Gomez were in full zombie-fighting mode filming. The 26-year-old former Carrie Diaries star was spotted wearing a blood-stained white T-shirt, while Gomez was dressed in a red T-shirt, also bloody, and short denim shorts.
After signing autographs for fans, the "Back to You" songstress returned to shooting on the second floor of an old roadside motel.
As for how she's dealing with Bieber's engagement news, a source told ET that the 13 Reasons Why producer is unaffected by the 24-year-old pop star's relationship with Baldwin, 21. Gomez and Bieber had dated on and off since 2010, but called it quits earlier this year.
“She does not care at all,” the source said. “What they do with their lives does not impact her. Selena is in such a great place and is surrounding herself with good positive people, and Justin and Hailey’s happiness doesn’t take away from her own happiness.”
The source continued: “Things just roll off her back now. She has a much better perspective on life now that she’s removed herself from the Hollywood bubble."
Here's more on Gomez's reaction to Bieber and Baldwin's romance: