Gage is the son of author and journalist Nicholas Gage. He was born in New York, and grew up in Athens, Greece, and then North Grafton, Massachusetts. He attended Brown University where he majored in American Civilization.

Gage wrote The Breed and the 2002 film Teenage Caveman.

Since joining the comic book industry in December of 2004, he has written a number of series including a 2006 Union Jack mini-series with Mike Perkins.

Gage wrote a comic book series based on The Man with No Name for Dynamite Entertainment and wrote some fill-ins for Thunderbolts for Marvel, with three one-shots before, starting with #122, writing the four-issue tie-in with Secret Invasion. Also at Marvel he wrote House of M. Gage wrote the "Secret Invasion" tie-in War Machine: Weapon of SHIELD and with the "Dark Reign" tie-in, The Initiative, he will become the main writer for the series.

At Wildstorm he wrote Prime with Darick Robertson and then worked on a number of Wildstorm titles, Armageddon, Revelations and the forthcoming Worlds End which allowed a number of titles to be relaunched.

Other recent or upcoming projects for other companies include Absolution for Avatar Press, G.I. Joe: Cobra at IDW Publishing. and Area Ten at Vertigo.

Gage is married to Ruth Fletcher Gage, with whom he often collaborates in screenwriting projects.

TV and film

• The Breed (2001)
• Teenage Caveman (2002)
• Special Victims Unit:
• "Mercy" (2003)
• "Ritual" (2004)
• Numb3rs: "Bones of Contention " (2005)
• Tomb Raider: "A Complicated Woman" (with Jim Lee, 2007, forthcoming)

Comics

• Deadshot (with pencils by Steven Cummings and inks by Jimmy Palmiotti, 5-issue mini-series, DC Comics, 2005)

• Legends of the Dark Knight (DC Comics):
• #201-203: "Cold Case" (with pencils by Ron Wagner and inks by Bill Reinhold, 2006)
• #214: "Superstitious and Cowardly" (with Phil Winslade, 2007)

• Union Jack (with pencils by Mike Perkins and inks by Drew Hennessy, 4-issue mini-series, November 2006 - February 2007, Marvel Comics, tpb London Falling, 96 pages, July 2007, ISBN 0785121811)
• Worldstorm (with Doug Mahnke, 2-issue mini-series, Wildstorm, 2006—2007)

• Post Human Division #1-12 (Wildstorm, 2006-2007) collected as:
• Volume 1 (with Doug Mahnke, tpb collects #1-4 & 6-7, 160 pages, July 2007, ISBN 9781401215002)
• Volume 2 (with Matthew Dow Smith and Andy Smith, collects #5 and #8-12, 144 pages, April 2008, ISBN 1401216781)

• X-Men #1-3 (with Andrea Di Vito, mini-series, Marvel Comics)
• Midnighter #8 (with John Paul Leon, Wildstorm, June 2007)
• Conquest - Quasar (with Mike Lilly, Marvel Comics, September—December 2007, tpb, January 2008, 96 pages, ISBN 0785127186)
• New Line Cinema's House of Horror #1 (DC Comics, September 2007)
• Avengers (with Mike Perkins, 5-issue limited series, Marvel Comics, November 2007)
• The Authority: Prime (with Darick Robertson, Wildstorm, 2007, tpb collects The Authority Vol. 4 #5-11, Titan Books, August 2008, ISBN 1845768612, Wildstorm, July 2008, ISBN 1401218342)

• Thunderbolts: Secret Invasion (168 pages, Marvel Comics, March 2009, ISBN 0-7851-2394-6) collects:
• "Breaking Point" (with Brian Denham, one-shot, January 2008)
• "International Incident" (with Ben Oliver, one-shot, February 2008)
• "Reason in Madness" (with Ben Oliver, one-shot, May 2008)
• "Running the Asylum" (with Fernando Blanco, 4-issue Secret Invasion tie-in, Thunderbolts #122-125, September—December, 2008)

• The Initiative #8-35 (with co-author Dan Slott and art by Stefano Caselli and Harvey Montecillo Tolibao, Marvel Comics, 2008—2010)
• Armageddon (with various artists, 6-issues, Wildstorm, tpb, 144 pages, April 2008, ISBN 1401217036)
• Revelations (with co-author Scott Beatty, and art by Wes Craig, Wildstorm, 2008, tpb, 296 pages, July 2008, Titan Books, ISBN 1845769333, Wildstorm, ISBN 1401218679)
• The Man with No Name: The Good, The Bad, and The Uglier (with Wellington Dias, Dynamite Entertainment, 2008)
• Wildcats: World's End #1-18 (with Neil Googe and Trevor Hairsine, Wildstorm, 2008—2009) collected as:
• Wildcats: World's End (136 pages, August 2009, ISBN 140122363X)
• House of M (with Andrea Di Vito, 5-issue mini-series, Marvel Comics, September 2008)
• Iron Man: Director of SHIELD #33-35: "War Machine: Director of SHIELD" (with Sean Chen, Marvel Comics, 2008)
• Absolution (with Roberto Viacava, Avatar Press, August 2009, ongoing)
• X-Men/Spider-Man (with art by Mario Alberti, 4-issue mini-series, Marvel Comics, 2008—2009)
• G.I. Joe: Cobra (with co-author Mike Costa and art by Antonio Fuso, 4-issue mini-series, IDW Publishing, March 2009, forthcoming)
• House of M: Masters of Evil (with art by Manuel Garcia, 4-issue mini-series, Marvel Comics 2009-2010)
• Avengers Academy #1- (with Mike McKone, ongoing series, Marvel Comics, June 2010—present)