Just like the Jungle Juice achievement, the Gold Rush achievement demands you to collect all (100) Gold Bars spread throughout the single-player storyline of the game.
100 Gold Bars is quite a number so you may have to spend some extra time before you can achieve this goal. Good thing is that after your first play-through, you can load any chapter you want and search for these gold items without any worry of missing any item.
And if you are having trouble in spotting any of these Gold Bars in the game, the following video can help you in finding them.
I will recommend you to complete the game once and then play each at Cadet difficulty to get these gold bars without any hassle. You will find it very difficult to get this achievement/trophy during your first play-through.