This spooky map from Steam user CWardee is known as the "Blackbrook Asylum". Perfect for a potential zombie survival game mode, RP server, or hide and seek style games within Garry's Mod, this creepy mod makes a typical game all the more frightening.
CWardee describes this map as a 20th century derelict insane asylum that has been overgrown by nature. According to comments, it's filled with plenty of spooky props, too. So player beware.