1)Since from the date of purchase you are in peaceful enjoyment of the property as absolute owner. Till today no one has questioned the Sale Purchase nor any one has challenged the said GPA is not valid. Therefore your absolute possession over the said property is proved. Therefore the said GPA is not required.
2)As you told you has lost GPA, for safety purpose you can file a complaint in jurisdictional police station informing of missing of GPA. This is only to maintain the record track.
3)One the sale is took place, the purpose of the said GPA automatically become end. After the sale purchase you are become the absolute owner. For verification of tittle over the property the buyer may demand the same. Other wise not required..