You get a pat on the back if you give yourself one.
Other than what he said above you dont get rewarded and you do have to gain back your attachments. However when you prestige it twice and then unlock the gun permanently you have all of the attachments unlocked instantly
I try to time my second weapon prestige with my push for 150 kills without attachments or perks. Three birds with one stone! After you prestige it, you lose attachments, but you'll usually get your reflex/laser/fast mags back with one game. It's not much of a loss.