Does Golurk's "Superpower" attack apply Resistance to the +20 damage it does to itself?
Yes, as long as Golurk is Fighting Type at the time of the attack, Resistance would be applied.
Superpower





x2

SqueezeFlip a coin. If heads, this attack does 20 more damage and the Defending Pokémon is now Paralyzed.



SuperpowerYou may do 20 more damage. If you do, this Pokémon does 20 damage to itself.
Does Golurk's "Superpower" attack apply Resistance to the +20 damage it does to itself?
Yes, as long as Golurk is Fighting Type at the time of the attack, Resistance would be applied.