Anvuss will make a one use feather fall item. He'll reclaim his gear, pick up Kharas, run to the edge, and jump off. He won't trigger the feather fall item until he sees ground coming up from beneath him.
If it's a truly infinite freefall, that ought to give him time to figure something else out... while it's no guarantee, Anvuss figures that it's more likely that this plane bears at least a passing resemblance to the physical laws he's accustomed to than it is that his attackers have some understanding of justice or mercy. Capture is to be avoided unless the alternative is certain death.
It occurs to him that it might be safer to try and retreat back through the gate, but he acts decisively. (Hopefully, this won't turn out to be decisively hurling himself and Kharas to their doom!)