Yes you are correct. As the appartment is purchased and registered before the introduction of GST then no GST is applicable. During such period, service tax was applicable and that too was also linked with the payment of installments. Completion certificate is the criteria common in both the tax regime. Developer has no right to claim GST from you as he rendered services to the landowner and not to you.
The payment had been made and registration was done before the construction of the flat then it was the liability of the landowner to collect and pay.