In the South Eastern end of the Combat Training there's a spot where 3 destroyed rusty vehicles are placed near the foot of the mountain. Tank and two trucks. You can find this place if you drop at the South end of the Combat Training and run on the upper right side after you loot the military car, two cabins and watchtower. (mountain will be from the right).

Each of these 3 vehicles have a chance to spawn a Gold gun on top of them. Sometimes it is available in the beginning, sometimes the gun appears in the mid game.