While Pokemazter is completely right about offense beating Gothitelle, I believe that he missed a key point as to why Gothitelle is so threatening. Shadow Tag Gothitelle means that Stall is virtually impossible now in UU (or any lower tier for that matter). I am going to touch base on the two moves which makes Gothitelle so deadly in my opinion, Trick and Tickle.
We all know how Trick works, switching items is always an annoyance, especially for any stall team. However, most well built Stall Teams usually have Trick Fodder, which means that they are able to play around a majority of Trick Users. However, the difference with Gothitelle is that you literally can hand pick what Pokemon you want to cripple with Trick, and unless your opponent is running Shed Shell, they have absolutely nothing they can do about it. And while you can say "ok, so more Pokemon run Shed Shell, no big deal", there are some Pokemon (Gliscor, Qwilfish, Snorlax, etc) who all really need Leftovers to function, and they cannot afford to run Shed Shell or they lose out on a significant portion of their walling capabilities.
Trick also works on teams that want to eliminate Rapid Spinners. A Spike Stacking offensive team with Gothitelle can literally defeat every common Rapid Spinner in UU. Hitmontop loses to Psychic, Blastoise loses to Thunderbolt, and Claydol loses to ShadowBall. Hitmontop has Sucker Punch, but you can just spam Trick until the PP runs out, forcing them to lock onto a move that you can abuse.
Trick also doesn't mean you can cripple Stall, it can cripple every single type of play style. Tricking things like Iron Ball on set up sweepers that are forced to stay in, or things like that make it so frustrating for the opponent, and unless they make EVERY single one of their Pokemon able to beat Gothitelle, then something on their team is guaranteed to be screwed.
Now there's Tickle. Simple concept enough, Tickle + Pursuit. Pokemon locked in on a poor move choice (Scarfed Heracross on Close Combat) easily gets sent down to -4 to -6 and gets demolished by Pursuit. We've seen this combination with Wobbufett, and we all know how good it was, so I am not going to dive into this more.
Gothitelle also has other moves that can beat other Pokemon. While Tickle beats Physical attackers, it also has access to Mirror Coat, meaning any choiced Special Attacker that can't OHKO Gothitelle (it has good bulk) is going to die.
I could go on, but I think these are valid enough points as to why I think Gothitelle deserves to be banned