uncle bought plot through GPA. Can he now register it in name of son?

sir,My uncle has paid 6lakhs while purchasing plot in 1995. The vendor gave GPA to sell a particular plot. Now he is planning to register that plot. Questions are : 1) Can my uncle go ahead and register this on the name of his son? 2) Do we need sign of the person who gave the GPA to my uncle during registration time ? 3) If my uncle registers on the name of his son, do we need to mention the sale consideration, because since its between father and son, there will not be any cash exchange? 4) Would it be legal sale if no money consideration is mentioned?