was this made in gbstudio
Viewing post in GameBoy Doom tech demo comments
Nothing that complicated. Your 360 view of the map is a top-down 2D scene that can wrap around itself and when you go forward the scene changes.
The gun is just a big sprite that plays a sfx and the animation when you press a button.
And finally, the hud is a part of the background set on a parallax layer that doesn't move, if you want to make a working one you'll probably have to draw some text on that layer that keeps track of the player's health, ammo and other stuff while Doomguy's head would be a sprite.