I wouldnt use those. While sand can be passed through the digestive tract in small quantities, I wouldnt trust sani chips to do the same. Id just pick up a bag of jurassic sand or repti sand or just use some paper towels instead.
Actually I'm not so sure, it wouldn't surprise me if impaction occurs in the wild too, in the rare location that their "dry grassland" environment actually contained sand, should they eat it. It's similar to the fact that snakes eat live rodents in the wild, but they aren't immune to any possible risks from doing so--in that case, scars, open wounds, or fatal injury.
Anything that will provide a flat surface for the gecko, and not allow feces to get down under it (so I would look for rocks that fit together in squares, for example). An additional advantage is that, if you use a nice rough rock, it should help wear down their sharp little nails.
Just whatever you do, you absolutely cannot put a ball python on sand, definitely. It's bad for them for a great many reasons.