Just dumping it into a keg won't get it evenly mixed & it won't pull everything out.
Also you should use the gelatin before carbing. I add mine & slowly carb it up so it's never under high pressure. By the time carbonation is reached, the gelatin has already done it's job. Gelatin will work in carbonated beer, sorta, but not all that well.
Either rack on top or use enough pressure to get a good seal & shake the keg to get the gelatin evenly mixed. Racking on top is just a lot less work.