GST applicability on ready to built flats

I booked a fully furnished flat in Noida by paying 65% payment on 15.9.2016, balance 35% was to payable on offer of possession. Possession offered on 22.10.2017. Completion certificate obtained by builder on 18.10.2017. Builder demanded and forced us to pay SGST and CGST at 12% each on balance cost and also on advance 1 year maintenance/club charges electricity connection. They also demanded interest on delayed payment. When we asked for the proof of payment of SGST/CGST against our flat and also interest made by them to SGST/CGST authorities, they are unable to provide proof. They are also selling the flats after getting completion certificate on 18.10.2018 without GST. What action can I take against the builder when they are unable to prove that they have paid the received SGST/GST to authorities. The builder says that where part payment has been made for booking of flat prior to introduction of GST on 1.7.2017, the balance payment attracts GST which has replaced ST. Under the circumstances, please suggest course of action. Should I file a complaint for anti-profiteering or consumer protection.?