It should stay in liquid/gel form for at least 24 hours. It sounds like you are using enough water but you could try slightly more. How is the humidity in the enclosure? Low humidity will cause it to dry out fairly quickly (but high humidity will cause it to mold quickly).

What kind of dish are you putting it in? Something not too wide but with slight depth is good (not wide enough/deep enough though that the gecko can get into it and drown if it is a hatchling). I use condiment cups for my geckos: http://www.acemart.com/concession/di.../prod5834.html. For hatchlings and young juvies I use the lids as there is much less waste but the food does dry out faster in these since there isn't much depth: http://www.acemart.com/concession/di.../prod5837.html.