GST on registration of flat

I purchased a flat and paid in full in 2013 from a contractor. Flat value was 20.40 lakh. I take possession. but I did not registered the flat in that year. Now in this year i.e. 2021 i am registering the flat. The contractor told me that I hv to pay GST for this. Please advise me whether I hv to pay GST to the contractor?