From Niantic's own update:
You’ll also earn extra Candy while you’re out trick-or-treating with your Buddy Pokémon: Candy rewards from catching, hatching, and transferring Pokémon will be doubled, and your buddy will find Candy twice as fast during the Halloween celebration!
(emphasis mine). So probably you will not get double candy for giving a berry at a gym, but the other you will get double candies for.