Gray has often collaborated with fellow writer Jimmy Palmiotti on series such as Jonah Hex, Power Girl, 21 Down, Uncle Sam and the Freedom Fighters and The Resistance. The two also co-wrote No Way Back, an original graphic novel to coincide with the release of the film. Currently, along with Palmiotti, Gray is writing the monthly Jonah Hex and Freedom Fighters for DC Comics, as well as the miniseries Time Bomb for Radical Publishing.

In addition he has also worked comic books based on the video games like Prototype, as well as writing the screenplay for the Dead Space prequel animated film Downfall.

Comics

Comics work includes:

• The Resistance (with co-author Jimmy Palmiotti and art by Juan Santacruz, IDW (originally Wildstorm); Collects #1-8; 192 pages; April 2009 (originally 2002), ISBN 1-6001-0407-X)

• 21 Down (with co-author Jimmy Palmiotti and art by Jesus Saiz, Wildstorm); Collects #1-7; 176 pages; November 2003, ISBN 1-4012-0120-2)

• Jonah Hex vol. 2 (with co-author Jimmy Palmiotti, DC Comics, ongoing series, January 2006—present) collected as:
• Volume 1: Face Full of Violence (collects Jonah Hex #1-6, 144 pages, Titan Books, December 2006, ISBN 1-84576-408-0, DC, September 2006, ISBN 1-4012-1095-3)
• Volume 2: Guns of Vengeance (collects Jonah Hex #7-12, 144 pages, DC, April 2007, ISBN 1-4012-1249-2)
• Volume 3: Origins (collects Jonah Hex #13-18, 144 pages, DC, November 2007, ISBN 1-4012-1490-6)
• Volume 4: Only the Good Die Young (collects Jonah Hex #19-24, 144 pages, DC, April 2008, ISBN 1-4012-1689-7)
• Volume 5: Luck Runs Out (collects Jonah Hex #25-30, 144 pages, DC October 2008, ISBN 1-4012-1960-4)
• Volume 6: Bullets Don't Lie (collects Jonah Hex #31-36, 144 pages, DC, April 2009, ISBN 1-4012-2157-7)
• Volume 7: Lead Poisoning (collects Jonah Hex #37-42, 144 pages, DC, October 2009, ISBN 1-4012-2485-1)
• Volume 8: Six Gun War (written by Justin Gray and Jimmy Palmiotti; art by Cristiana Cucina; 144 pages, collects Jonah Hex #44-49, DC, April 2010, ISBN 1-4012-2587-X)
• Volume 9: Counting Corpses (written by Justin Gray and Jimmy Palmiotti; art by Darwyn Cooke, Dick Giordana, Jordi Bernet, Paul Gulacy and Billy Tucci; 160 pages; collects Jonah Hex #43 and 50-54, DC, October 2020, ISBN 1-4012-2899-2)

• Uncle Sam and the Freedom Fighters (with co-author Jimmy Palmiotti, 8-issue limited series, DC Comics, September 2006 - April 2007, tpb, July 2007, ISBN 1-4012-1336-7)
• The Beginning (with co-author Jimmy Palmiotti, art by John Higgins, graphic novel, Fox Atomic Comics, 2007, ISBN 978-0061243547)
• Uncle Sam and the Freedom Fighters: Brave New World (with co-author Jimmy Palmiotti, 8-issue limited series, DC Comics, November 2007 - June 2008, tpb, September 2008, ISBN 1-4012-1954-3)

• Terra (with co-author Jimmy Palmiotti and art by Amanda Conner, miniseries, DC Comics; collected as Terra with Supergirl #12; 128 pages; December 2009, ISBN 1-4012-2510-1)

• Power Girl (with co-author Jimmy Palmiotti and art by Amanda Conner, ongoing series, DC Comics, July 2009—present) collected as:
• Volume 1: Power Girl: A New Beginning (April 2010, ISBN 1-4012-2618-3)
• Volume 2: Aliens & Apes (October 2010, ISBN 1-4012-2910-7)

• No Way Back (with co-author Jimmy Palmiotti and art by Tony DeZuniga, original graphic novel, DC Comics, June 2010, ISBN 1-4012-2550-0)

• The Last Resort Graphic Novel (with co-author Jimmy Palmiotti and art by Giancarlo Caracuzzo; 120 pages, IDW, March 2010, ISBN 1-6001-0615-3)

• Random Acts of Violence Graphic Novel (with co-author Jimmy Palmiotti and art by Giancarlo Caracuzzo; 72 pages, Image, April 2010, ISBN 1-6070-6264-X)

Films

• Downfall (with co-author Jimmy Palmiotti, 2008)

Video games

• Mortal Kombat vs. DC Universe (with co-author Jimmy Palmiotti and others, 2008)

Interviews