I don't necessarily think you need to hard counter RG other than with, perhaps, another RG to keep him stun locked so he's not stunning your team (omega Daka is another option). Other than that, the best counter to RG is simply rendering him useless through proper planning then making his auto-taunt work against him.
Plo Koon, especially as leader, seems like it would be good against Leia teams running Nute Gunray as lead (PK lead grants passive stealth removal for every ally turn and the Nute lead is a bonus crit chance buff to stealthed heroes while having a basic that may proc stealth for himself). It's highly situational, but being prepared for any team you need to face in arena is never a bad idea. Unless you're already sitting on a very healthy and deep roster, there are probably heroes that will serve you better in more capacities than PK.
However, if you are in dire need of a dispeller (or secondary dispel to back up QGJ), Asajj comes to mind and would probably serve you better in gw and/or raids.