Bill Paxton also stars, with Dan Gilroy directing.
Story follows a driven young man who discovers the nocturnal world of L.A. freelance crime journalism. Rahm will play a news channel editor.
Tony Gilroy, Gyllenhaal and Jennifer Fox will produce along with Michel Litvak and David Lancaster, who are producing and financing via their Bold Films banner.
Rahm will return for the upcoming season of “Mad Men.” He is repped by Gersh and Forward Entertainment.