As the land owner had executed the GPA in the favour of the another person which was a registered one which means the power of attorney is an irrevocable one where , the GPA when executed through a sale deed is valid if it is a registered one.
The sale was done by executing the sale deed , then the deal that is done in selling the property remains valid., as the GPA has been executed in form of registered general power of attorney, the power of attorney is valid even after the death of the executant and you can still get the property registered on the basis of said GPA.
The sale deed will be valid , but you need to have documents which you will need to consider while executing the sale deed with death certificate of land owner ,encumberance certifiacte and this agreement should contain the terms and conditions agreed upon between the parties, and binds them. An agreement to sell is the basic document which should be an agreement to sell in writing.
then there will be execution of a sale deed. This agreement is signed and executed by the seller and buyer on a non-judicial stamp paper. then you will need to pay stamp duty.