Yes, he's OP (not sure about "borderline broken"). He's used in 66% of #1 teams, and CG should consider such low diversity a problem. People who have maxed Shoretroopers are using him in Empire teams instead even though he has no synergy. There's also a new low synergy team that is becoming popular: GK (L) Chaze DN R2.
He's not the most OP- DN, Chaze, and maybe Thrawn are the most OP characters and need nerfs more than him, but he's still way to prevalent.
Team composition and mod strategies are supposed to be big parts of the game, so it's unfortunate that the meta is dominated by a small fraction of its characters and that speed secondaries are the dominant factor with mods.
Regardless of how iconic a character is or how hard they are to gear, they should never stifle diversity as much as GK.