For the Free Gun code, I believe they are:
(from Gamespot)
PS3
at the main menu screen
(Hold R1) Triangle, Down, X, Left, Square, Right, O, Left, Left, Right, Square
360
When you are customizing your character. (Changing guns + clothing)
(Hold RB) Down, Down, Up, Up, X, B, X, B, Y, Up, Up, Y.