Hi Bala,
Hope you are doing well !!
Yes, you are liable to pay GST as the booking is done before receiving OC.
The GST is applicable, when the complex, building or flat, is sold before its completion and the consideration, whether in full or in part, is received before its completion. So, even if you book a flat, where the builder asks you to pay just one per cent or a very nominal payment and the balance after possession, you will still have to pay the GST on the full amount. Conversely, if the entire sale consideration is paid after the building is completed, there is no GST liability.