The Golden Machine Gun (Normally Written as The Golden Machine Gun on the Steam Forums) Was originated by the user Noodleboy347 (who is now banned) on October 4th, 2008.
According to legend (a few posts) The Golden Machine Gun is obtainable by completeing all achievements (364 so far), This legend has many times, been proven wrong. The Golden Machine Gun has been made so popular, it even got its own Steam Group .

It is slang which is often used on the steam forums when someone thinks of some ridiculous idea.

Since Team Fortress's Comic (Loose Canon) was released, some users on the forums have speculated about The Golden Wrench (which is actually a hidden item ingame) and The Australium Machine Gun (Source). This meme doesn't really spread about much, but is normally found in the TF2 section of any forum.

Much like The Golden Wrench's ability, some forum goers say that the The Golden/Australium Machine Gun will turn players into gold statues.

This meme will only be found on TF2 forums (or very rarely, /b/), and the orignal idea came from Ian Flemings "The man with the golden gun".

On August 31st, the steam user WiNGSPANTT and 11 others, deleted their golden wrenches for charity, which raised $30,202.41; Unfortunately 2 of them deleted their wrenches a few days befo, because of technical difficulties.

Also, on the 30th of September, 2010, VALVe added in the Polycount Update paints, one of which was "Australium Gold", to paint hats. In an interview, VALVe said that soon players will be able to paint their weapons golden, The Golden Wrench may soon be coming to a game near you!