normally prolonged exposure to low 80's (under 85F) shouldn't be too stressful for healthy adult crested geckos but it may have reached higher temps than that which you weren't around to observe. For young geckos, any prolonged exposure to temps over 80 can affected them much faster.
So far what you did was the best thign to do, get the temperature down as soon as possible, soak the gecko in cool water and give it the opportunity to drink all it wants. Not sure if ice on teh top of the tank would help but since cold air drops, it may be more effective than I know.
Moving the cage to the lowest point you can (ie floor) can expose them to temps lower than having them in a cage a few feet higher. A shadow dish with some ice cubes will give them some cool water to drink (one or two cubes with lots of water around them is best, we don't want any gecko tongues stuck to ice do we?).
For now, closely monitor the temp of the gecko's cage, keep him as stress free as possible and wait. If he was cooled off quick enough he should pulll through. If his heat stroke was too bad, he won't make it but only time will tell. I don't even know if a vet can increase the odds of survival or not. If you know one good with reptiles, you can call and consult with him for other suggestions.
-----
PHLdyPayne