NVIDIA Quadro P6000 draws power from 1x 8-pin power connectors.
Since 1000 watts power supply already has 8 Pin (6+2) power cables, there is no need to use HP adapter cable.
Simply connect G1 8 Pin to the GPU as shown below:
It is not necessary to use G1 to G4 in order, you can use any available pins from G1, G2, G3, G4.
After installing GPU as described above, make sure to securely lock the PCIe bracket which holds the PCIe card on motherboard firmly. Make sure cables are not protruding in any of the fans inside the cabinet.