Yes the developer would collect GST from you and this would be collected when he transfers possession to you and GST would be @5%.
If you sell apartment before completion certificate you need to collect GST from customer and deposit it with government and it can be a cumbersome task so try to sell after completion.
You are transferring 60% of rights to developer so he will collect GST on 60% of value of land because for him the consideration he received is 60% of land.
Capital gain will also be calculate at the time of transfer of possession and sale consideration would be fair value of flat received and you can claim exemption u/s 54F since you are transferring land.
Yes GST will be recovered by builder in any case.
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