Her father, an American residing in Los Angeles, California, was the son of Lithuanian-Jewish immigrants and made a career as an actor.
Gillian was a member of the famous dance troupe, The Bluebell Girls in Paris in the 1960s, who then went into modeling and performing in television shows.
Her first part out of acting school was to play Shelley Bower in the Liverpool-based television drama Brookside in the summer of 1999.
Wescourt settled into her new life stateside and found roles in television commercials before landing the part of Gwen in Bug.
In 2006 she landed the role of Dr. Baden in the TV thriller Watch Over Me, before appearing as a news anchor in an episode of Ugly Betty in 2007.