What does are the requirements for grand theft auto? Is it a certain amount of money the vehicle costs or the classification of vehicle? Would something like a small moped which doesn't require a motorcycle license get it or would it have to be something where you need a license?

Theft laws are purely state laws in the US, in fact many states do not have a "grand theft auto" law, it is merely considered theft, or armed robbery or car jacking depending on how it was done.

So this is depend on how the terms of that law is written within your state. It normally will have to do with how the car was stolen, and the states meaning of the word auto ( wich would have been worked out over the years of court cases.

If the moped is registered or can be, it could fall within that law. Depending on the state and other factors