Is a sale deed executed by the GPA holder without the knowledge of the principal valid?

Mr.Santosh paid some amount in a society and hence was allotted a plot in a society in 1990. The registration of that plot was not yet given. He gave an unregistered GPA(was made notary) to Mr.C.R.Reddy to take care of the registration of the said allotted plot. In 1994, the society executed a registered sale deed of this plot to Mr.Santosh. In 1994 itself, Mr. Reddy without the consent and knowledge of Mr.Santosh Registered the GPA which was written in 1990 and thereby executed a registered sale deed selling away half of the property to Mrs.M.Sudha. 1) Now, can Mr.Santosh cancel that GPA unilaterally? 2) Can a GPA given in 1990 be registered and validated after four years? Can this GPA be used to execute Saledeed? 3) As it was all done without the knowledge of Mr.Santosh(principal), Can he cancel the sale deed executed by Mr.Reddy? 3) It came to be known that Mr.Reddy and Mrs.M.Sudha expired long back, so does that GPA and Saledeed get cancelled and that half of the property also can be claimed by Mr.Santosh?