Wait until you are at the end of the level. When you board the Gondola, look behind you and in front of you should be a large pillar. Go there and jump up onto it. Climb up, and when you reach the top, be sure to stay there for a few seconds. A very strong force will throw you back down. When back on the ground, you will hover rather than walk, and a pulling force will stop you from going towards the front of the Gondola. This force is unbreakable and you cannot jump. This is probably an invisible wall that is to prevent you from being left behind as the gondola moves on.