The game automatically gets a 5 star for seriousness because this is definitely the funniest one I've seen by far. Great ending, but inevitable because THE HOUSE ALWAYS WINS.
I have lots of feedback for this game because I genuinely liked the gameplay a lot, and I feel a bit betrayed by how quickly it ended.
What you did well:
The story is extremely funny, I don't know why I find it funny but it just is.
Being able to gamble mid-fight using your earned money and I'm assuming the luck stat to get different and better weapons mid-fight is extremely genius and I'm not sure why other games aren't doing something similar.
Having it so only certain weapons can kill the boss forces you to target minions and try out more than 1 weapon, and additionally giving limited boss-damaging ammo is also a fantastic way of balancing strong weapons.
I like the music.
The bullets hitting enemies and the boss are satisfying.
My suggestions for you:
There is a bug in your game regarding the full screen. I'm not sure if this happens with anyone else but if I exit full screen then re-enter full screen with f4 my game will crash. I know why this happens since I also use GameMaker. You have to make it so any time you're running draw functions or full screen functions etc. that you first use an if statement: " if surface_exists(application_surface) = true " then your draw/full screen code.
It appears that the player can't die? I didn't fully test it but I didn't see me having a health bar and I didn't see any consequences of getting hit.
This game could go down the path of being a story game or an endless game (or both with enough time. I highly recommend you consider expanding on it.
Have powerups or some sort of collectibles spawn around to encourage the player to explore your map more.
The boss' spawn could be delayed since it seems that he just immediately joins the room with the player.
Definitely more UI things like damage numbers to see how good your weapons are and if they're doing anything.
Overall, great job! This game has a nice charm to it and I honestly like it a lot.