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
Icy WindThe Defending Pokémon is now Asleep.


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.