"Occasionally attacks twice" != Definite >50% DA
Not to mention that in order to get that occasional DA, you are stuck using that one Nyzul WS. We don't know anything about the GKT WS other than it's the only GKT WS that's 3-hit and it at least APPEARS to be darkness-based. Limiting yourself to a single WS on SAM cuts off opportunities for closing SC's that others open, thereby reducing your total dmg output.
Sadly, I won't be able to test it out myself, since the Nyzul Polearm is far sexier than the GKT (in appearance), and so that's what I'll be upgrading.
But to sum up: the Nyzul GKT doesn't appear to give anything more than what you can currently get via other means with a GKT that would do far more dmg (thanks to the 2.5x dmg) and at the same time not limiting you to a single WS to receive the benefits. Hence, not broken.