I wouldn't do an adult crested gecko in a 12x12x18, personally; it's on the small side for a adult to really have enough room to jump and climb. A juvenile or a sub-adult would do fine but you'd want to upgrade once the critter reached full size.

I don't know for sure about gargs but I imagine the adults have similar size requirements to cresteds, they're relatively similar species in terms of husbandry IIRC.