No. That is not the reason. The reason is a spoiler so you'll just have to wait and find out.
Its interesting that you think G0-T0 is the one to beat though. Perhaps I underestimate him.
-is Invisible (in multiple ways)
-is Very hard to kill (for Force Users too)
-is Skilled at operating in the underworld
-has No planets to defend. Or a base. Completely offensive
-his Power base is literally built of bounty hunters and assassins
Basically, it doesn't matter how large the opponents army, because G0-T0 doesn't care about planets. He doesn't care about warfare. He'll stay hidden. His countless assassins will aim straight at the opposing leader. GOOD assassins. And if the other army attempts to find him with bounty hunters of their own... well, easier said than done.
The Kaggath is all about killing the other guy. The endgame. And G0-T0 is built for endgame.