Rap God is the 31st floor, where the player can explore a trashed, secluded floor of an apartment building and talk to one of the five interactable people.
The floor was inspired by a map called ph_apartments in the Garry's Mod Steam workshop. It was supposed to be the whole map, but it could take too long to develop and too much time to get to the next floor according to one of its floor developers (not one of the main developers).
You can see how well-armed they are when you visit the room with the black shirt, and obviously while you try to ask for one, he'll simply refuse.
The five people you can talk to each have their own limited dialogue.
According to one of them (the purple shirt), there were 24 people as a group, but due to an unexplained attack (the brown shirt) and mysterious disappearances, they were down to 5 people remaining.