The five gold bars in this mission are hidden at the following locations.

Directly above the west gun you must destroy there are a group of X spots.
The island in the middle of the lake has a gold bar and an X spot with a collectible gun.
After you get the goggles and go to a weapons depot for some ammunition, there will be three tanks. The gold bar is behind the tanks on the golf course up the hill.
When you go around the mountainous rocks that separate you and the other pathway, you will reach the top of the map. At the tip of the map is an X and a tank. The gold bar is next to them. A gold bar is at the rear of the mansion's bottom floor.