so i just read the dev blog and investigate a little bit. apparently you can go to a gallery room (the dev said it's a debug room) by interacting with a TV inside your bedroom 10 times... there's no indicator that you doing it properly, just spam action button on the TV and it should work.
the only annoying thing about this room is that the boss/special girls don't look the same when you meet them at playthrough, the sprite are different and they not in order you encounter them or anything, you need to remember their position by yourself.