You now have the option to change Trubbish’s forme at a facility in Kalos called the Waste Treatment Plant. You can collect 17 items across the region and bring it to the plant where a worker will stuff the item into a Trubbish, changing its forme.
Here is a list of the items you can fill Trubbish with and the types that it becomes:
Dead Leaves (Poison/Grass)
Dead Bugs (Poison/Bug)
Water Bottles (Poison/Water)
Popsicle Sticks (Poison/Ice)
Ashes (Poison/Fire)
Batteries (Poison/Electric)
Rocks (Poison/Rock)
Dirt (Poison/Ground)
Bird Beaks (Poison/Flying)
Boxing Gloves (Poison/Fighting)
Manure (Poison/Normal)
Bed Sheets (Poison/Ghost)
Sunglasses (Poison/Dark)
Crystals (Poison/Psychic)
Soda Cans (Poison/Steel)
Snakeskin (Poison/Dragon)
Glitter (Poison/Fairy)
It is unknown if the items are consumed upon use or if you can make multiples of each forme, but once the Trubbish is filled you cannot revert it back to pure Poison. It will learn different moves while leveling up depending on its secondary type and Garbodor will also have the same type as its pre-evolution.