Since balloons only pop when you get attacked
Their balloon wouldn't pop, but they would still come down, because the move is simbalizing gravity strengthening, I have used gravity on a pokemon holding a balloon before and it worked.
http://www.serebii.net/attackdex-bw/gravity.shtml http://bulbapedia.bulbagarden.net/wiki/Gravity_(move)
these are just links to more info on the move, it doesn't say anywhere in them whether or not it makes pokemon on balloons vunerable to ground moves, but I have tested it and it worked.
