I can't even imagine how much work you put in to this, I mean to make a 3D top-down shooter in a 2D game maker.
Though, I would like to see this game made in GDevelop, my opinion is that you picked the wrong engine and make it more difficult for no reason. You could get the same and even better results in minutes using a 3D engine like GameGuru and the free FPS Creator assets for example.
FPS Creator (free assets on the bottom of the page)
I believe there was a MOD for FPS Creator to make top-down shooter games, it might be no longer available though but the assets from FPSC still very useful for making such games in GameGuru. Especially the wall, floor, door and window segments.