➤ Guzma speacializes in bug Pokemon.
When you battle Guzma at Aether paradise, he'll have a team consisting of his Golisopod, Ariados, Masquerain and Pinsir. Bug types are weak against Flying, Fire and Rock types; so your Toucannon and Gyarados would be great in this situation. Make sure you have a decently strong flying move; and you'll be fine.
The Gyarados you have can also do some decent damage, but keep in mind that Guzma's Golisopod is a water type too.
I strongly suggest to keep your Decidueye/Oranguru/Umbreon away from this battle, since bug is super effective against grass/physic/dark types. I don't know the smeargle's movesets, but I'm sure it can manage some damage too.
If you truly need to, you can switch one of the Pokemon I just said with Zygarde just for this battle, since Zygarde is a ground/dragon type.