i wish i had gotten to this before the ratings ended.  this was very fun overall and i think your sprites are really cute!  definitely has a GameBoy feel to it.  i love the music and sound effects!  the levels were nicely designed and you have spike hazards and a couple of enemies to add a bit of variety to the gameplay.  i largely agree with everything danwithfaintpraise mentioned.  

it would be nice if the stationary enemies had some kind of sprite change to telegraph when they are about to pop up and fire.  you can figure out the timing easily enough, but it's a touch that really can help players and make the game feel more fair.

the jump needs some work, and makes the game a little bit harder or more frustrating to play than it should be.  i think most players will agree that a platformer with a good arch to the jump just feels better.  so, like Dan mentioned, you want the jump speed to gradually diminish as the player reaches the peak of their jump and then let gravity kick in and take them back down to the ground.  even though it's using Unity, i think this video explains things pretty well.  i know there are some great jump tutorials out there for GameMaker as well, so you might want to check out some of those.  i think this HeartBeast tut covers it.

also because your jump goes up and then back down so quickly, it makes it virtually impossible to jump over the enemies.  maybe that's something you wanted in the game, but it was a little frustrating for me.  having a jump that has a little more air-time would also solve this issue.

i like the boss a lot.  it has a nice attack pattern.  i found the bullets to be a bit difficult to dodge due to the jump.  i think they wouldn't be an issue if you changed it a bit.  i also agree a little more feedback on when the boss is getting hurt would help.  i wasn't sure if the face change meant i was hurting him at first, but that's great start.  if you look at other retro games, often times they will add some kind of flashing on the sprite when they're damaged, or an impact effect of some sort.  also a sound effect for when there's actual damage could help as well.  i realize this is a jam, so you may have just run out of time for these things.  i didn't get any sounds into my game, lol!

i'm still using GM1.4, but i assume version 2 has about the same options when exporting.  when you go to "create application" to export there a few options to "save as type" below the file name.  i would definitely agree to not do the installer. 

anyway, sorry for the wall of text!  i hope this is helpful feedback for you!  

I really liked this! A couple of nitpicks though:

• I don't know exactly what the options are in GameMaker Studio 2, but in GameMaker Studio 1 you are able to export a game either as a standalone .exe or as a .zip file (containing the .exe), rather than as an installer. I think for jam games especially it's preferable to have something a player can just download and run as opposed to have to install it.
• I don't think the issue with the jump is that it was too "floaty," per se, but rather that it's too fast, as vallian88 said. You'd ideally want your jump to slow as you reach your max height, which I'm not sure if yours did. That said, in a short game like this with small rooms, it wasn't a game-breaking issue.
• Sometimes when I clubbed the final boss he'd make a face, and sometimes he wouldn't, so I wasn't sure if I was doing something wrong or if there was some hitbox inconsistency.

All that said though, I had a lot of fun with this and you should be very proud of it as your first completed game. I liked how you made different branching paths to explore, the shooting enemies were thoughtfully designed with their limited kill-able window of time, and having the 5, 10 and 15 melon gates was a nice way of dividing the game up into distinct sections. Great job!

Game feels good and chill. Graphics are cute tho!

Music is great but feels not "gameboy" to me. Also the jump is kinda strange to be honest, even if playable.

Oh and I had some strange issues with enemies losing their colliders for some reason after I come back to a room I visited.

Overall, cool game!

Really great music! Simple cute gameplay. Your jump is a little fast compared to your fall. Great job.

Some of my favorite music of this jam by far! love the color palette too! Might wanna give a tiny bit of extra jumping headroom if you keep working on it. But great job! And again I loved the sounds.