Nice game, especially since its your first game! Some feedback though:
-maybe add some text at the start showing what button you press for the gravity mechanic because I got stuck not knowing how I would get up to that high platform for a while,
-also, maybe make it so that you can change you velocity more when you jump and your in the air, just to make the platforming more precise.
But just me being picky, it was a cool experiance and the visuals, atmosphere and music fit together really well, keep it up!