If you are going to buy a flat which is under construction from a builder who are selling flats then you have to pay the GST 18% as per the GST Norms. If the flat is ready to occupy then the builder might already paid the same where he will addon such GST cost with building cost in respective flat.
If you are buying a flat from Individual owner who are not a builder then you need not pay any GST but you have to pay the Registration charges applicable depends on the State Government and their norms.
Registration of deeds are done in the Sub-Registry offices as per Guide lines/instructions contain in Indian Stamp Act, 1899 with state amendments thereof, Registration Act, 1908, Tripura Registration Rules, 1954, Transfer of property Act, 1882, Indian Conveyancer and notification of govt. in revenue department on table of fees.
According to Tripura you have to pay 5% stamp duty.