Using Gallade as an example if a Gourgiest uses Trick or Treat which typing is removed? Or will it become a Psychic/Fighting/Ghost type?
>Trick-or-Treat adds the Ghost type to the target, in addition to the Pokémon's original type(s). If it is used against a dual-type Pokémon, that Pokémon will have three types at once.
So, it'll become a tri-type - Psychic/Fighting/Ghost. :D
Hope I helped. :) Source