Gellman's acting debut was in a 1998 episode of the television series, Animorphs. After appearing in several more television productions, he was cast in supporting roles in the 2000 film, Urban Legends: Final Cut and in 2002's Jason X. In both films, his characters eventually became victims of serial killers. He appeared in 2002's The Matthew Shepard Story as Pablo, the love interest of the title character. Gellman became known among teenage audiences after playing the part of Paolo Valisari, an Italian international pop star, in 2003's The Lizzie McGuire Movie.
Most recently, he has been cast in the Canadian series Monster Warriors. He plays a lawyer, Rafe Torres, in the long-running soap opera, The Young and the Restless. 10 years ago, it was revealed that the character of Rafe Torres was gay. Gellman has also appeared as a model in numerous print ads, including Omega watches. He made a cameo in Degrassi Goes Hollywood, as well as playing Pete in multiple episodes of Greek. He has recently played the role of police officer, Garrett Reynolds in hit TV show, Pretty Little Liars. He has also starred in the hit TV show 90210.