Strategically placed magazines work wonders. Place them hanging 1/3 - 1/2 off a shelf, whenever a cat tries to jump on to the shelf they land on the magazines which give out from under them. Cat goes tumbling down. Usually gets point across after the second or third fall - depending on the cat.
You can also try a few empty pop cans around the edge of a shelf, the noise from those being knocked down is a good deterrent.
Look for some cheap metal lid locks if you don't have a couple of them already. I occasionally have problems getting them off so there's no hope that a cat can.
You can also use solid paper or some of the decorative aquarium backing to put around three sides - so the cats can't sit next to the cage and watch through the sides.
Remember to be consistent in making them get down and telling them no, try to discourage the cats from even staring at the gecko cage from the floor. ...I've been able to teach cats that geckos are off limits, but not the counter tops.
No chemicals. Reptiles are a lot like birds in their sensitivities to chemical fumes - this includes candles and air fresheners too. It does not take much to mess up their nervous systems so it's better not to risk it. If you want something that is stinky to cats to use as a deterrent, you may be able to set some orange peels next to the cage.