Player 1 (farthest ahead) plays ballast 10 Player 2 (in the middle) plays sleeping gas Player 3 (one space below player 2) plays ballast 11
Does player 1 get to move? The rules state that the gas would cancel the 11, but does that mean it does not work, implying no one moves, or does it disappear all together making player one's 10 the highest.
Sleeping Gas: "This card cancels all cards (except the Super Motor) which have been played on the level immediately below the player playing this card. Balloons more than 1 level below, and those on the same or higher levels, are not affected. Canceled cards are discarded." (from the PDF scan of the rules located at http://boardgamegeek.com/filepage/4991/montgolfiere-pdf )
The last sentence contains the key to resolve your dilemma. Player 1 gets to move. Rise up! And so on to the moon.