Removing gauge cost from Onslaught would arguably be a nerf because the DPS gain would theoretically be balanced around by removing potency elsewhere. That is, our final DPS would stay the same, but we would now have to Onslaught on cooldown, reducing its availability as a gap closer.

The "issue" with Onslaught right now is that the gauge to potency ratio has changed, but Onslaught received no potency increase to compensate for it.