Power of attroney

No, he cannot sell.  On the death of your uncle the GPA will become infructuous. Also check up whether there are sale powers in the GPA or not.  Some times only limited powers will be given to contest court cases,  to represent before revenue authorities and governemntal organisations etc.  

Power of attorney is an authority given by executor to represent him or do some act,things and deeds for on on behalf of the executor.IN case there is no authority given by your uncle to sell the property, the attorney holder can not sell the property for an on behalf of your uncle,the executor.If the executor dies, the power of attorney becomes infractuous,invalid. In simple words, it also dies on death of executor who has given the same. For any clarification/assistance, you may contact me .You may THANK if  you like reply.

if principle dies automatically power of attorney is invalid document to use