In 1997, Edward Kirilian used The LB-7 laser prototype and the three Toltec Skulls to cause a cross-dimensional rift to the Netherworld. He planned to enslave more entities. Egon Spengler managed to interfere with the portal's stability. The ghosts returned back to their home.
There are four ghosts that appear in the Gateway which are all alternate colored ghosts from the Extreme Ghostbusters Intro. The same ghosts also appear as extras when Spirit Slaves fly into the Gateway too.