For the US Army uniform, finish the "Lights, Camera, Action" stranger mission. Then it will be added to your list.
The only costume unlocked via preorder was the Deadly Assassin costume, this is not available to non-preorders. The golden guns were also preorder only, however they can be obtained in multiplayer. --- "A hero, forever loyal to the flames of war, rests in Outer Heaven 193X - 1999"
gentlemans atire is unlock via linking your gamertag with the rockstar social club, its free though, its a fancy suit that allows you to play high stakes $250 poker in the blackwater hotel on the second floor behind a shinier door.