A fifth round pick by the Astros in 1999 from Long Beach, CA, Gallo spent four seasons in Houston as a lefthanded relief specialist. He compiled a 4-3 record with a 4.11 ERA in 160 games.

Gallo also pitched for Houston during their playoff runs of 2004 and 2005, posting a 1.69 ERA in 10 postseason games. He was released after the 2006 season and tried to catch on with the Blue Jays and Rockies but did not appear again in the major leagues.