Gastineau is the daughter of Lisa Gastineau and former New York Jets player Mark Gastineau. During the divorce proceedings between her parents she lived with her grandmother and attended public school at Clarkstown High School North in New City, New York. After graduating from high school she attended the University of Alabama before returning to New York City to model.
After moving back to Manhattan Gastineau signed with the Elite Model Management, and became an in-demand model as part of their celebrity division. Within weeks of signing with the agency, she appeared in a spread for Stuff magazine. She walked runways for Anne Bowen and Anna Sui during NYC fashion week in 2005, and signed an endorsement deal with L'Oréal Paris, which led to a European advertising campaign. She also became the face of high end casual wear designer Ed Hardy. She appeared on the cover of Steppin' Out, Hamptons and Lucire magazines. In 2005, she appeared on Maxim magazine's Hot 100 list, ranking at number 60.
In 2006, she again walked the runway for Fashion Week in NYC, this time for designer, Yumi Katsura, and modeled for the designer again later that year, showcasing her bridal collection.
Her modeling career continued to flourish, including contracts with MAC cosmetics, as well as runway shows for boutiques Heatherette run by Traver Rains and Richie Rich. She also shares a high-end accessory line with her mother, Tres Glam, and in 2007, they added a costume jewelry line as well.
Gastineau has continued to model and do numerous magazine layouts, which included the cover of Social Life magazine, and spreads for L.A. Confidential and Life and Style magazines. In 2008 at Fashion Week in NYC, she walked the runway for designer Tamara Pogosian, and was featured in a campaign for Ferrari California in Beverly Hills.
In 2004, Gastineau landed a development deal with the E! Network and began production on a reality series which would also feature her mother Lisa Gastineau, an elite Manhattan socialite, and ex-wife of legendary NFL defensive end Mark Gastineau.
In 2005 the series, eventually dubbed The Gastineau Girls, premiered on February 15 and became a modest hit for The E! Network. The first season chronicled mother and daughter's quest for love in New York City as well as Brittny's formal, successful introduction into the world of modeling. On January 31, 2006, the series' first complete season was released on Region 1 DVD in the United States.
In between seasons of The Gastineau Girls, Gastineau appeared on another E! Network reality show, Filthy Rich: Cattle Drive, which observed the activities of spoiled celebrity children on a Colorado cattle ranch. The show was produced by Joe Simpson, father of Jessica and Ashlee Simpson. Gastineau later appeared on the NBC game show, Identity, and guest hosted episodes of The View and Isaac Mizrahi's talk show. She appeared on Lifetime's Top This Party, E!'s Keeping Up with the Kardashians and MTV's popular programs The Hills and Cribs. She also hosted segments for Fuse TV and Spike TV.
In 2009, Gastineau appeared in a small segment of the movie Brüno.
