I haven't try this I need somebody to confirm this.
Very easy tutorial of how to get any weapon in gold.

Requirements:
Have at least one weapon with gold camo.
Have your account linked to CoD: Elite

1) Set in Custom Class 1 the weapon you have in gold.
2) Then in MW3 go to the main menu and is time to switch to the PC.
3) In your PC go to Elite and sign in.
4) Go to your Custom Class 1 and switch the weapon you have in gold for the one that you want in gold.
(Now this is the tricky part)
5) Now in MW3 (you have to be as quickly as possible) go to Multiplayer>Xbox Live>Create a Class>Custom Class 1>Weapon with gold>Any Proficiency>Any Attachment>Camo, do NOT select a camo
6) Now the Elite thing should pop-up to accept changes. Click on YES.
7) Select the gold camo and you should have the weapon that you want with gold in gold.

This is similar to ImYoDaddySon tutorial but instead of a proficiency is a camo. So if this worked credits go to ImYoDaddySon.
ImYoDaddySon Post: Forums/t=3103875/new-proficiency-...t-tut.html