Do I need to prestige the weapon twice to get gold? Or can I just leave it at the first rank?
You do not have to prestige to unlock gold. You do have to prestige it 2x to unlock the "weapon master" emblem for that particular gun.
However, once you start working on the 150 kills with no attachments/perks... that's the time to prestige it. You'll have the attachments back by the time you finish the kills with 2xXP on weapons.