Life orb on Galvantula ; Swap energy ball for giga drain, the recovery will be very much welcome for the LO Recoil ;
Curse Ferrothorn might look fancy, yet with the amount of chandelures out there and salamences with flamethrower, your not gonna do much with it. Might like to consider leech seed or something.
Gigalith's good on paper. But sturdy wont save you from the omnipresent Breloom that'll 2hit k.o bullet seed you in 1 turn, making it easily a 6v5.
I also see no situation where Payback would be better than Curse Gyroball Stab on Ferrothorn.
Sky Attack might look fancy, but a one shot trick pony wont do much for you, specially if they have a physical wall like skarmory. What you COULD do is add Brave Bird for a more consistent way to deal damage, slap in maybe a Choice Band or a Life Orb, and maybe consider adding Superpower and / or U-Turn. I see no moment where you would shadow claw instead of brave birding anything.
I would advise replacing Foul Play (Cause first of all, if anyone uses dragon dance, you wont live to tell the story) on Zoroark for another move. I see no reason to invest in attack points just for low sweep, when foul play does not depend on YOUR own attack, but on the opposing pokemon's attack. Could be a substitute, to keep playing the illusion game for longer and surviving priorities leaving unharmed.
Like its been mentioned above, if you really want to use Gigalith might aswell put a Custap Berry on it to guarantee the explosion comes out.
For golurk, you have the huge speed weakness and the fact golurk can die in any water / ice move should let you down in many situations. You have nothing to stop a faster foe from killing you, drain punch has no stab, and i doubt you'll ever K.O anyone with just one. Try the No-Guard set with Dynamic Punch and Earthquake (maybe stone edge and rock polish too, or shadowpunch), you get a little better coverage and with luck the confuse might aswell save your life a couple of times.