Well apparently Gearbox didnt make all of it, but an ex dev for GB said most of it was by Timegate (Section 8) and on the other hand you have Randy Pritchford (Gearbox) said 80% was in-house ..... probably to stop people cancellig pre-orders if they heard who was actually making it :)
http://www.eurogamer.net/articles/2013-02-12-aliens-colonial-marines-primary-development-outsourced-to-section-8-developer-timegate-studios-report?utm_source=eurogamer&utm_medium=email&utm_campaign=net-daily TL:DR
Comments from an ex-Gearbox developer dug up by Superannuation suggest that the majority of the game was outsourced to TimeGate Studios, developer of downloadable console shooter Section 8: Prejudice and various FEAR expansion packs.
"Hate to say it, but I wouldn't get your hopes up too high for Colonial Marines," the ex-dev wrote last year. "I used to work at Gearbox, and the development of that game has been a total train wreck, going on what, six years now?
"Gearbox isn't even making the game, except for the multiplayer. Primary development was outsourced to TimeGate Studios, which has a less than stellar past."