Kraken and the others activate once per board state. As soon as the next step in the cascade happens, their trait can trigger again. So Gar’Nok will happily work as many times as you can consecutively fire him off.
Edit: should work, not will work. We already have evidence that how people expected Gargantaur to work is not how he apparently does. So there is a definite mismatch between expectations and reality.