The problem is that GG can pretty much tech anything. Mewtwo Lv.X, Giratina, Azelf Lv.X (goodbye obvious psychic counters), Mesprit, Dusknoir, Nidoqueen... The list goes on. It's pretty much the only deck consistently getting the techs out due to Gardy's power (even the Lv.Xs thanks to Teleportation).
KOing it early on can also prove hard if they just get their Roseannes in place. They can usually give up 2-3 prizes and still come on top of most decks due to locking other decks in place and taking cheap prizes in the lategame.
Generally I use Uxies to cause problems to GG myself. They do 50, go under the deck (not KO'd back, and also helps against Dusknoir) and whenever Plox ceases, you can get them back and draw to 7.
Still, Plox is nothing to underestimate and will definitely be seen at Worlds once again. Hey, the deck almost won Finnish nats Masters division three times in the row (placed 2nd this year, won 2 previous nats), so it's still nothing to underestimate.