Regigigas is terrible in PU.
Offensively, as a Normal type, it is outclassed by Heliolisk, one of the best Pokemon in PU. Heliolisk doesn't have to burn 5 turns to actually do something, and it has lots of immediate power and some cool new toys like Surf and Volt Switch. Defensively, Miltank is a better Normal type thanks to its great Ability in Thick Fat which lets it stave off threats like Charizard and Choice-locked Sneasel, while it has useful support moves in Heal Bell, Milk Drink, Stealth Rock, and some other options. Regigigas is extremely predictable and thus can be stopped very easily (Heliolisk and Miltank's Smogon set is the more common set, yes, but even if the enemy knows what set they are, they can still fall prey to something like Volt Switch or Toxic, while Gigas is very predictable and does not really have anything to make up for this), and has the worst ability in the game which resets upon switching. This along with the fact that it flat-out loses to Passimian, Scrafty, and Toxicroak makes burning 5 turns much more difficult than it seems.
Oh, and if ZU has piqued your interests, Regigigas sucks ass even there.