Can you pawn a gun not registered in your name?

That depends on where you live. In most of the US, there is no registration required for firearms- dealers must retain a record of a sale. If there is a local registration requirement, you may not possess the gun at all if not registered to you. If no state registration requirement exists, yes, you may pawn a gun that you own- IF the pawnbroker has a Federal Firearms License. Some do, some don't.