GST and Stamp Duty are 2 different things. As per the GST law, the projects under construction attracts 12 % get of the basic cost of the flat. The builder is liable to collect the 12% GST from the flat Buyer of an under construction flat and pay it to the Govt.
On the sale of land GST is not levied because a sale of land attracts Stamp duty.
stamp duty is concerned it is a respective state govt tax and while registering the sale agreement you have to pay stamp duty as per the ready reckoner rates of the respective states and registration charges.