Paper towels work best for juvies as no chance of impaction, but an adult can be on eco earth. But it gets dirty quickly so it needs to be changed often. THe nice thing about paper towels is that it is easy to clean and easy to monitor feces. a 48 gallon tub is kinda on the small size, thats only 12 gallons and these guys really do best in 15 gallon tanks or larger, but if it is the only size you can fit into a space it will work so long as stated your gecko can be provided with a photo period.
i think paper towels are best or some sort of non loose substrate as it lessens the possibility of impaction and you can see easily if its been eating by seeing poop and its much easier to keep clean and less chances of mold forming. as far as a 48gal tub i am not sure i dont know what size that really is. but it would be best to have a clear tub and make sure it has plenty of ventilation. Good luck. Smaller geckos need smaller spaces and moved into bigger as they get bigger.