x2Reactive PoisonThis attack does 60 more damage for each Special Condition affecting your opponent's Active Pokémon.
Gastro AcidThe Defending Pokémon has no Abilities until the end of your next turn.
My active Arbok uses its "Gastro Acid" attack against Weezing with the "Aftermath" Ability. Then on my opponent's turn he uses Weezing's "Smogbank" attack to do 20 damage to each of my Pokemon. If I have Latias-EX with the "Bright Down" Ability on my bench, would it take damage or not?
Since Weezing has no Abilities due to Arbok's attack, it would do 20 damage to Latias-EX.
Arbok's "Gastro Acid" attack says the Defending Pokemon has no Abilities until end of your next turn. Does retreating get rid of the effect so you can use Abilities?
Yes, it does.
Arbok's "Gastro Acid" attack says the Defending Pokemon has no Abilities until end of your next turn. Is that referring to the opponent's next turn or the attacking player's next turn?
It refers to the end of the next turn of the player who used Arbok's attack.