An interesting take adding a mouse controlled gun, visuals are serviceable and are not intrusive. All in all a good attempt at the genre. Even adding hidden blocks which you can find by firing the gun is quite unique.
Some player feedback is definitely needed, it can be hard to tell when you have made a mistake as well as what level you are on. The lives are a good indication but I feel perhaps should have been in a place where players would be more aware. Some rudimentary audio would have gone a long way to improve the player experience. Some of the platforms on the 4th(?) level seemed out of reach and a clear solution couldn't be seen otherwise.
Some touch on polish and tweaking would result in quite a strong submission, and it is submitted relatively early. Good work.