Good Morning,
If you are purchasing it after the seller(even though it's a landowner flat) has got the certificate of completion then GST is not liable to be paid, if you have paid any advance before the seller occupancy certificate then the GST has to be paid.
1) In your case, as the agreement is completed in the month of February and the completion certificate is yet to be received, the GST is applicable and you have to pay the GST.
2) The GST should be paid to the person who is selling the flat to you, that means you have to pay tax to the landowner who is selling the flat to you.
Thank you