He created and wrote a science-based series for ITV, Eleventh Hour, starring Patrick Stewart as a government science investigator and advisor.
Gallagher's series format was acquired for a US television remake by the CSI trio of CBS, Jerry Bruckheimer and director Danny Cannon.
Life Line, broadcast in 2007, was a two-part supernatural mystery starring Ray Stevenson, Joanne Whalley and Jemima Rooper.
Gallagher was later lead writer and story supervisor on NBC's 13-part series Crusoe, screened in 2008/2009, and contributed two episodes to the US version of Eleventh Hour including Medea, the season finale.
Legacy, a two-part story for season 16 of the BBC's Silent Witness, was Best Drama winner in the 2013 European Science TV and New Media Awards.