Hi
You don't need to pay any Capital gain taxes.
The investment made in Oct 17 shall be eligible for claiming exemption under section 54.
Though Capital gain calculation needs to be done and shown in ITR, the same has to be shown as amount reinvested in another residential house property and thus claiming exemption under section 54.
You may repay the outstanding loan by paying 6 lacs. You may even utilise it anywhere else. Exemption is nevertheless granted as reinvestment has been made.