I liked the proposal! I see that a lot of effort was put into that graphics, nicely done! I've some suggestions and recommendations that I think will improve the game experience. Small changes that makes a lot of difference.
- In the game maker, you can export an executable for your game, making your game accessible for those who don't have Game Maker installed. You go into Build->Create executable and choose the type of package you want (a zip file or a installer);
- Teach the players how to play the game, by giving them instructions or making the level design intuitive;
- If the character shoots, get hit or hits an enemy, put some correspondingly sound effects (in the internet, search for websites that have sound effects in public domain, in case you can't make them);
- Its not a rule but... I'd advise making the art for the protagonist before making the enemies. Because the player will be playing with the protagonist, so it's important!
Good job!