Television:
* 7th Heaven (1996-2007) as Simon Camden
* The Pretender (1999) as Young Jarod
* The Division (2002) as Aaron Whitman
* Summerland (2004-2005) as Bradin
* Without a Trace (2005) as Nick Donovan
* Cold Case (2005) as Tommy Garrett
* CSI: Miami (2006) as Kyle Decker
* The Secret Life of the American Teenager (2008-2013) as Ricky Underwood
* CSI (2010) as Chris
* Supernatural (2010) as Zachariah
* The Glades (2011) as Jimmy
* The Client List (2012) as Blake
* Twisted (2013) as Danny Desai
* Hawaii Five-0 (2014) as Danny
* The Young and the Restless (2017-2019) as Joe Clark
* The Rookie (2020) as Detective West
Movies:
* The Little Rascals (1994) as Spanky
* Honey, I Shrunk the Kids: The TV Movie (1998) as Nick Szalinski
* The Ultimate Christmas Present (2000) as Danny
* The Princess Diaries 2: Royal Engagement (2004) as Joe
* A Christmas Wedding (2006) as Matt
* Solstice (2008) as David
* The Shortcut (2009) as Tyler
Other:
* The Wonder Years (1993) as Kevin Arnold (voice, episode "The Secret")
This is not an exhaustive list, but it covers his most notable roles. It's worth noting that David Gallagher continues to be active in the entertainment industry, so he may have additional projects that aren't listed here.