Love the visual! 

"be as harsh as you want, I need criticism so I can improve"

This is by no means a bad start, I will try to be as helpful as I can here:

To start, you can export an executable file by going to "Build/Create Executable" in GameMaker. I can't entirely remember if that's available with the free version, but I'd reckon it at least lets you export to zip with GX.Games so you can upload it as an HTML5 build. 

I have some criticisms of the game itself, and also of some of the conventions in the project file itself, since this is a rare instance of someone actually providing the repository of their project: Try to stretch your collision instances as much as possible, the current level alternates between tiling them for each collision and stretching them. The way you have collisions set up means that it is just plain better to stretch them. Say you made a level that was much bigger than the current one, it would start lagging because of the instance count. So less collisions = better in your case currently. 

Some additional advice would be to not layer your collision below the tiles, but to instead toggle "visible" in the object itself. This makes it so that you can put the collision above the tile set in the editor, so you can clearly see where gaps and issues are. It would have helped catch the missing piece of collision in the 3rd room.

I can say that for the most part, the sprites are actually pretty good and coherent, and the colors pop out and have good contrast and aren't blending together.

I'd like to say more but most of it is subjective stuff, I just wanted to cover all of the important things that I know would definitely help anyone in your situation. I hope this wasn't too harsh or hard to understand, I can really appreciate the value in a project like this as a first submission to a jam. You've got a solid foundation and understanding, stuff will get better over time! :)

Is it possible to make it playable in browser or executable? Don't have Gamemaker installed.