Apparently, Gunn is not a lock yet, but has emerged as the first-in-line choice. Others who have been in the running (and are apparently not yet fully discounted) include Peyton Reed ("Bring It On," "Yes Man")
and "Half Nelson" directing duo Ryan Fleck and Anna Boden, which speaks further volumes about Marvel's willingness to take great risks with unconventional directors who have little to no tentpole experience but clearly possess other talents they the studio believes they can leverage (reps for Fleck and Boden tell us this is untrue).
Newcomer Chris McCoy (a 3-time Black Lister with no produced screenplays thus far) was recently hired to write a script with a "quirky comedic bent" which should suit Gunn just fine. Gunn most recently directed the twisted and dark and grossly underappreciated super-hero/vigilante indie "Super" in 2010 starring Rainn Wilson. He's also known for writing 2002's "Scooby-Doo," Zack Snyder's "Dawn Of the Dead" remake" and directing the 2006 science fiction-horror-comedy, "Slither."
'Guardians' follows a five-member interstellar team including Star-Lord, Rocket Raccoon, Groot, Gamora and Drax the Destroyer (learn more about the 'Guardians' team here) on an undisclosed adventure, though considering the team's ties to Thanos, the purple-skinned villian revealed in "The Avengers" post-credits scene (see that sequence here), we assume the film could be a gateway picture to Marvel's big daddy of them all, "The Avengers 2" in 2015.