Also, an early gameplay video revealed that you get the chrome Hammerburst skin by having the "Rookie Gear" Achievement in 2 (reach level 5 in multiplayer).
According to an unofficial statement from one of the devs, the gold and chrome weapon skins will be based on historical/past Achievements in previous Gears games.
I think we can assume the other skins like flame and urban camo will be unlocked the same way they were in the beta.