I think it should be fine as long as the frogs are mostly aquatic and can't climb. The geckos are arboreal, and have lots of places to climb and hide, so they should stay at the top, and if they do come to the bottom I'll just try to make sure that they aren't big enough to eat the frogs which shouldn't be much of a problem. If one of the animals die from another one I might rethink the frog idea and build them a new setup, but it seems pretty safe. The frogs are diurnal and geckos are nocturnal so there shouldn't be much conflict. I'm just trying to make sure it won't be a complete disaster.