Really neat to see a rhythm game made in GameMaker with this level of functionality. In the game the skulls were at the bottom of the play area instead of at the top, like in the screenshots. There was also no feedback to let the player know how accurately they're hitting notes. If you're going to develop this further, I'd recommend prioritizing some sort of player feedback - whether that means a fancy particle effect or the usual perfect/good/miss/etc. indicators.