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.
Source: Ancient Origins FAQ (2015-08-13), TPCi Rules Team (2015-08-13)

Relevant cards

Note: All applicable cards may not be listed below.
XY7_35