Gage broke into the comic book industry in December 2004 with the DC Comics miniseries Deadshot. One of his earliest Marvel Comics works was a Union Jack mini-series with Mike Perkins. For Wildstorm Productions Gage wrote The Authority: Prime with Darick Robertson. His subsequent Wildstorm work included Wildstorm: Armageddon, Wildstorm: Revelations and Wildcats: Worlds End which was part of a relaunch of a number of titles. During Marvel's "Civil War" storyline, he wrote the best-selling tie-in book Iron Man/Captain America: Casualties Of War. He also wrote the miniseries World War Hulk: X-Men whose first issue sold in excess of 85,000 copies. Gage wrote the tie-in book Avengers: The Initiative, co-writing with Dan Slott beginning with issue #8 and eventually becoming the sole writer for the series with #20. He continued on through the conclusion of the run with #35. Spinning out of that series' storylines was Avengers Academy, which Gage launched with artist Mike McKone. That series introduced several new teen characters to the Marvel Universe and ran for forty issues.
In March 2008 Gage wrote the four-issue miniseries G.I. Joe: Cobra for IDW Publishing. That same year he wrote the first seven issues of The Man with No Name for Dynamite Entertainment, which stars the iconic Western character portrayed by Clint Eastwood. The storyline is set after the events of the sequel The Good, The Bad and The Ugly. Later that year he wrote the creator-owned series Absolution for Avatar Press, which focuses on superhero actions after he develops post-traumatic stress disorder.
In 2010 Gage wrote the original graphic novel Area 10, a crime thriller about an emotionally disturbed New York police detective who, while on the case of a bizarre serial killer, begins to exhibit psychic abilities after his head is impaled by a screwdriver.
In 2011 Gage was approached to write Angel & Faith, the canonical continuation of the adventures of Joss Whedon's Buffyverse characters, as part of Buffy The Vampire Slayer Season Nine. The title was nominated for a Harvey Award for Best New Series.
In July 2012 Gage published the original graphic novel Sunset, through Top Cow/Minotaur Press, a noir tale of an aged mob enforcer facing the demons of his violent past. As of 2012 Gage and his wife Ruth were working on the historical epic The Lion Of Rora for Oni Press.
From 2011 to 2013 Gage and Dan Slott co-wrote issues 661 - 662, 664 and 695 - 697 of The Amazing Spider-Man. After that series ended with the death of Peter Parker, a new series, Superior Spider-Man was launched in 2013. Gage co-wrote 10 sporadic issues with Dan Slott. He also wrote the "Inhumanity" tie-in, Inhumanity: Superior Spider-Man.
From 2013 to 2014, Gage co-wrote issues 14 - 23 of Bloodshot and H.A.R.D. Corps for Valiant Entertainment.
In 2014 Gage and Dan Slott co-wrote two of the stories in the anthologized first issue of the relaunched Amazing Spider-Man, while their collaboration on the final arc of Superior Spider-Man ranked at #3 on the New York Times Paperback Graphic Books Best Seller List. In the same year, Gage and Angel & Faith artist Rebekah Isaacs took over the Buffy The Vampire Slayer title, beginning with Season 10. The first collection of that series charted at #10 on the New York Times Best Seller List for Paperback Graphic Books, while the second collection charted at #8 and the third at #10.
He also co-wrote Spider-Verse, and wrote Spider-Geddon
In 2016, with his wife Ruth, Gage contributed to the Eisner Award-winning Love Is Love (comics) anthology to benefit victims of the Pulse nightclub shooting and their families.
He also wrote two series following the 2019 Disney Pixar film Incredibles 2. The Incredibles 2: Crisis in Mid-Life! & Other Stories was published on March 5th, 2019 by Dark Horse Comics. Gage wrote the first two issues, while Landry Q. Walker wrote the other. The three stories happen after the film. Gage's two stories (Crisis in Mid-Life! and Bedtime Story) were illustrated by Gurihiru and J. Bone, respectively. The other series is The Incredibles 2: Secret Identities, which is a story that also takes place after the Incredibles movies by Brad Bird. The series focuses on the character Violet, and an edition with all three issues was released on October 1st, 2019, again by Dark Horse Comics. Gage wrote it with Jean Claudio-Vinci as an illustrator. A new series, focusing on the character Dash, will be partly released in February 2020, under the title 'Slow Burn'.