GST for resale flat without flat completion certificates from authority.

Hi, I am trying to buy resale flat in Pune. This building and said flat has not received completion certificate. Partial completion certificate is received for some flats and those flat owners received possession from builder. Society is also formed by those flat owners. First owner has not received the possession letter from builder. Individual flat 'Agreement of Sale' is there between builder and first owner in 2016. During that period GST was not there. VAT is paid first owner and entire consideration is paid by first owner to builder. I do not have to pay any charges to builder. This building is joint venture between first owner (landowner) and builder. First owner (Landowner) has got more than 10 flats in this building, and he has individual sale Agreement for all his flats. Now he wants to sale his flats as individual owner. My question is - As this flat (which I am buying) has not received completion certificate and possession letter is not there with first owner (landowner) do I need to pay GST? in first sale agreement total flat cost is 65 Lakh and I am buying it for 90 Lakh. Do I need to pay GST on addition cost of 25 Lakh? If yes to whom I have pay GST? Regards, Sanjay