From someone who knows how hard it is to make video games, I have to say that this is simply
not good.
I'm completely willing to ignore the terrible, eye-raping visuals, because even when I'm using Actionscript I have someone else do the art for me, but there is just something fundamentally wrong with how this game is set up.
Bullets should never spawn in the center of the character, I'm not familiar with GML but I know enough to know that you're not doing your math here. The projectiles also move really slow and firing the weapon feels like firing your gun in Space Invaders except less satisfying. The zombies have literally no AI, they'll keep running straight at you and won't even try to go around obstacles, and the hitbox on the player is screwed up so as long as you're facing the zombie, it can't get close enough to hit you.
The controls are also irky, not being able to strafe side-to-side (ergo, you not doing the necessary math again) or being able to walk backwards makes it feel incredibly awkward to play. It's like you have to stop in place to shoot the zombies and you can't shoot them and run away at the same time. I recommend you just bind W and S to Y movement and A and D to the X movement and use the mouse controls for aiming only.
If you've really had 6 years experience, there's no excuse for making something this terrible. If you're a 12-year-old kid like I was when I could do this with Actionscript, then you need to realize that you create impossible to reach expectations of yourself when you tell people that you're an expert.