If the builder is selling the flat after receiving the OC then no question of GST arises.
However if sale agreement is made before receiving OC and some consideration also has been paid then GST would come into picture at the rate 1% if cost of flat is less than 45 lakhs and area is upto 640 Sq Ft in metro cities and 960 Sq Ft in non metro cities. In all other cases the GST Rate is 5%.