Suction caps work because air pressure on one side is no longer counterbalanced if there is a vacuum on the other. But geckos' feet can stick in a vacuum where there is no air pressure, so suction cannot be the reason. This is the attraction between electrically charged objects, for example a plastic comb rubbed with cloth can pick up small pieces of paper. There are no skin glands to produce any. Keratin, the protein in skin, is too slippery.
Geckos can even stick to polished glass.
The best explanation seems to be that the geckos' feet can exploit the weak short-range bonds between molecules. That is, they stick via van der Waals forces . But for such weak forces to work, there must be an enormous intimate contact area between foot and surface, so that enough individual weak forces can add up to a very strong force. These spatulae can provide the necessary contact area. But they showed that an individual seta had an attractive force 10 times stronger than expected.