Theoretically? Well Manhattan has a population density of about 73,000 people per square mile so it's certainly possible to safely provide for 30,000 people in the space of Guantanamo Bay.
In practice? There's no way they'll be doing anything like that, and the cruelty is the point.
No, they can't. Guantanamo is a very harsh environment with very small capacity
Edit: well, I guess if the plan is to starve them and put them in unliveable conditions where disease is rampant and they begin to die in droves..yes it would make sense then.