The other weapons could be made golden with a cheat device. The message that describes it says that it is a paradox, and that this weapon doesn't exist in the database. Their logo would be similar to that of the Morph-O-Ray, but the background would be red instead and have a no sign on it. There is no improvement to them, other than that.
In Going Commando upgraded weapons are sometimes named gold weapons, probably because weapons' icons are gold.
Some weapons have additional upgrades added to them once they are Gold versions. The Blaster ricochets off walls and enemies, the Devastator can bounce off of anything to hit enemies up to three times, the Suck Cannon has an improved ammo capacity, the Morph-O-Ray turns enemies into giant chickens, the Tesla Claw can arc electricity to multiple enemies at once, and the Glove of Doom spawns giant doom bots.