The move from the Misawa Kawada match was Planned. Kawada had told Misawa at some point he was doing it.
It was a one off spot, and was NEVER called the Ganso bomb, it was called by it's technical name:
Jumping Kneeling Vertical Sheer Drop Powerbomb.
Thesz's Ganso Bomb was the Original Powerbomb (Ganso meaning Original) It's as Ace309 described, a Throwing Piledriver.
As for the Misawa Kawada spot, you'll notice Misawa's head isnt that far off the group, and he uses his arms to take some of the impact. Still nasty, but not that brutal.
It also didn not end the match. Not even close.