careful of which notes you play - the gate opens when you are at full health
assuming youre using a controller - three of the four face buttons are used to play notes (A,B,X on an X-box controller),
if youre using a keyboard - Space, K, L all play notes and have actions in the game.
Controls can be checked any time in game by pressing start (or enter) and tabbing over once, (right bumper, or "E" on the keyboard)