Gyro ball

[pokemaster1970]

Gyro Ball states: Switch this Pokémon with 1 of your Benched Pokémon. Then, your opponent switches the Defending Pokémon with 1 of his or her Benched Pokémon.
If my bench has a Deoxys EX, and my active Magnezone attacks with Gyro Ball for 80+10 on my opponents Yveltal EX, is the Active Yveltal EX Ko'd (weakness), or is the Yveltal EX switched before damage occurs?

Thank You

 

[PokePop]

Unless an attack specifically states otherwise, damage is always done first.
So damage, including Weakness, is done, then the switching occurs, then you process the KO of the Yveltal, in your example.