The song has to be at least 2 minutes long. A strategy you can use is to enable Ironmode, so you can restart every time you overfill. The score required for a Gold Medal is displayed next the Gold Metal icon on the loading screen or pause screen. With all characters, Clean Finish adds a 25% bonus to the score. To assist in getting a Gold Medal, ensure there are no blocks left on the grid when the song is over. There are other bonuses you can get which are specific to the character you are playing as. Ironmode automatically adds a bonus if you finish with it enabled. With Mono characters, never hitting a gray block earns you Stealth, which earns you a 30% bonus.