In addition to his role on 7th Heaven, Gallagher has kept himself busy by starring in television films such as The New Adventures of Spin & Marty opposite Judd Nelson, Angels in the Endzone opposite Christopher Lloyd and Walker Texas Ranger opposite Chuck Norris as well as Summer of Fear, It Was Him or Us, Bermuda Triangle, The Secret Keeper, A Dad's Life and Kart Racer.

Currently he can be seen on the big screen in Little Secrets, starring opposite Evan Rachel Wood and Michael Angarano. He is currently in pre-production on Coldwater, in which he will star and produce. It is scheduled to start filming in the spring of 2003.

Gallagher made his mark on the big screen starring in Phenomenon alongside John Travolta, Kyra Sedgwick, Forest Whitaker and Robert Duvall. In the hit sequel Look Who's Talking Now, he re-teamed with John Travolta to star opposite Kirstie Alley. You can also find him in the title role of Warner Bros. Family Entertainment's film Richie Rich's Christmas Wish.

Gallagher has been nominated three times for "The Hollywood Reporter's Young Star Award" in the category of Best Performance by a Young Actor in a Television Drama Series. David won this award in 1998 for his role in 7th heaven.

He currently resides in Los Angeles.