Gerbils only need about 1-2oz of water a day. Gerbils can go one day without any ill effect. If gerbils go longer than 24 hours without water they will start to show signs of dehydration. You may see them licking the wall of an aquarium or other surfaces trying to lick up moisture. They may dig under their water bottle, bang it around, or chew on it. Once dehydration sets in their skin will be less full, their eyes may be closed or sunk in, they may be listless, and/or cool to the touch.
Offer fresh water and wet foods like cucumber or grapes.
In extreme cases f dehydration they may need subcutanous fluids to be injected under their skin to rehydrate them.