1. Yes even than also capital gains tax will be paid by you. What you entering in is quid pro quo agreement i,e he will provide you apartments & in return you will give him land area for 4 apartments. Though their is no actual flow of money but still properties are being exchanged. Thus implication of Capital gain will arise. Since you are parting with Capital Asset i.e your land so capital gain on it needs to be paid. Question is what can be considered as selling price of land ??
1. Selling price of land can be considered as benefit you are getting (cost of 4 flat which builder will sell to others )or
2. you would have got if sold in open market. (can be obtained by valuation done by valuer)
Note: Capital Gain is part of Income tax - (Direct tax) & GST is indirect tax i.e consumption tax. Both needs to be paid & both are separate.
2. Yes Registration charges are not tax. It is done to register flat/Apartment in your name.