Urusla is not a part of the Gossip Girl novels.
Ursula makes her only appearance in Dan de Fleurette as a client of Serena van der Woodsen's employer KC Cunningham. KC introduces the two of them in the hopes that Serena can befriend Ursula. They become fast friends, despite KC asking Serena not to tell Ursula that her best scene in the upcoming movie Fleur is being cut. Serena does as she's told, but defies KC when she suggests that Ursula not cause a scene at the movie premiere. As a result, the two remain friends and Ursula tells KC that she will only continue to be her client so long as Serena still works for her.