If it's a ground dwelling gecko, a longer and wider tank is needed. I Use 20-40 Gal tanks to house my ground geckos(They were once upon a time fish aquarium, now converted into a reptile tank. If it is a climber and prefers to sit in the tree's / bushes you'll need a terrarium, which is vertically based rather than horizontally.
You do have a couple of options though. You may use a fish tank, buy a reptile terrarium, aquire a flexarium Or even 10Gallon + Storage containers( if its ground dwelling)
DO NOT USE SAND BTW
your gecko has a high chance of injesting the sand which in all ways possible is unhealthy. Again, i don't know what species you have but speak with somone about the substrate, DONT USE SAND!(OR ANYTHING THAT LOOSE)
Depends, if they need humidity a glass tank, or ventilation, a mesh tank. For heating uth, flexwatt or heatcable for the bottom of the glass. For substrate, Im not familiar with the species you want but generally you want a no loose substrate. The above poster exaggerated about the dangers but still I wouldnt use it.