For some reason, Blizzard put in weapons completed done in a gold skin that rewards people with a bunch of competitive wins. Like 3,000 points worth. Just like silver and gold portraits, all that means is YOU PLAYED A LOT. So oooh look that Widow has a golden gun. Big deal, they just played a lot and got enough points. It’s not an indication of skill on any particular class.
I personally haven’t done much competitive because golden “guns” doesn’t motivate me. First off, it barely looks any different at all on some skins. I believe Mercy in particular has a skin that already has a golden weapon.
The gold barely makes a difference on some characters, like here’s a Reaper example.
Regular weapon (Halloween skin)
It barely seems worth the work to me. Maybe if it had a Power Torrent enchant or something.