I had to read the comments to understand the gun can be used to fly. The character not rotating his arm to aim also makes it hard to recognize you can aim in any direction.
After realizing the gun's potential, I actually like the game concept.
I suggest making the character arm a separate sprite and rotate it to face the cursor. Also have a visible impact where the shots hit, or a visible projectile.
Since the gun is so good at flying, you really don't even need a jump button. Removing jump button might force players to realize you're supposed to use the gun to fly.
Playing in Chrome browser, I see a grid-like graphical distortion happen frequently.
The music is too repetitive and I found it a bit annoying. The sound effects are harsh on my ears.