EMS,
A 20long is not near large enough for this animal. The 40-50 breeder that someone recommended above is typically the very smallest recommended for uromastyx. I would suggest going back to the store to see if you can make a quick exchange/upgrade.
I disagree with those that feel a 10-gallon might be sufficient for hatchlings. I do not think a 10 or 20 gallon should play any roll in the keeping of this species unless you are using it to store excess substrate
Some might say that they can achieve a gradient in a 20 long. Heck, I could probably achieve a gradient in a shoebox- doesn't mean that it is a good one or one that is sufficient for these animals. The higher heat requirements including a basking spot of 120F and above (I typically run mine 120-140) make it next to impossible to get a ‘proper’ gradient which should not only include a good variance in temperature but also a good layout of space throughout the various zones.
I have kept quite a few geryi over the years and I would say this is one species that likes to roam about a little more than others. I think placing a 10” geyri in an enclosure this small will likely result in some major health issues and I would not be surprised if the animal succumbs to one of these issues. Not only do these animals like to roam, they tend to stress a little more than other species and/or handle stress differently. Combine an animal that stresses and likes to roam with a 20long aquarium and you have a recipe for disaster.