In the 1984 movie Ghostbusters, the characters of Dana Barrett and Louis Tully become possessed by the terror dogs Zuul and Vinz Clortho, respectively. The actual mechanism of how this happens is not shown or elaborated upon in the movie, but from what I can gather the hosts are "attacked" in some fashion and afterwards the terror dog and the host share one body.
This has always bothered me because the terror dogs are clearly corporeal (Vinz Clortho does some severe damage to Tully's apartment), so I cannot imagine that they just "enter" the host's body by passing into it like some full-floating vaporous apparition. When I was a child I had thought that the hosts were eaten by the terror dogs, but while that serves as good nightmare material for a 6-year-old I now no longer think that makes much sense.
Is there any information as to what method the Gatekeeper and the Keymaster used to possess Dana Barrett and Louis Tully? I would prefer official sources (something like the novelization, Word of God/Aykroyd, officially licensed material, etc.), but I'm OK if something like the cartoons or video games shines some light on this.