I agree that Zuruzukin will probably land in OU. It has many pluses with excellent STAB, decent movepool and ability, decent stat distribution, and semi-reliable recovery in Drain Punch. It is well-prepared to deal with a wide range of threats and with Tyranitar on the decline (at least in my experience; Hippo isn't nearly as screwed by Dory and Landlos), it is quickly becoming a secondary bulky Dark of choice. Not saying Tar sucks and personally I haven't used Zuru yet, but on some teams, it will function a lot better than Tar. Plus its DW ability is fucking Intimidate, which is kickass.
As for Gigigiaru (Klinklang?), I have a few problems with it. Gear Change is a great move, but Gigigiaru has a few 100% counters no matter how much it sets up. These include Swampert and nearly every Steel type. Gear Change is perhaps the only reason it won't fade into obscurity, but it's a problem when every other Steel, defensive or not, has infinitely better coverage than it. Its entire movepool consists of thirty-some moves....almost Wobbuffett level.