Gpa

Sir, I had purchased a site measuring 40'x60' at Mysore from a NRI through Indian GPA Holder (first party). While executing lease-cum-sale deed and Possession Certificate to first party from the govt agencies, the first party has authorised through GPA to sign the document such as Lease-cum-sale Deed and Possession Certificate (not for SALE) on behalf of the owner. This GPA copy or original is not available with me or with the first party. My question is if I want to sell the site this document (GPA) is necessary, if this is not available, what is the alternative ? Please advice me sir.