The Gilded Sword was the final form of the Kokiri Sword in The Legend of Zelda: Majora's Mask. Once you get what the swordsmith demands, he'll upgrade the Razor Sword one final time. Unlike the Razor Sword, this final upgrade is permanent and can't be broken.
I always loved getting the upgrade to the Gilded Sword, and I thought the gem on the sword's hilt looked pretty cool. It was basically Majora's Mask's equivalent to the Master Sword, which came in handy in combat later on.