The tutorial video comes from user nublet06 who says that if you wish to get the secret gun, you’ll have to throw a grenade at the guy who stands on the roof of the building inside the warehouse. This will make the guy run away and get blown up off the building, allowing you to pick up the gun and experience the game at a whole new level. To make things even easier, here’s the tutorial video about the secret Killzone 2 weapon: