NVIDIA Quadro P6000 draws power from 1x 8-pin power connectors.
Since 750 watts power supply has only 6 Pin power cables, HP 6 pin to 8 pin adapter cable must be used.
Connect G1 port (6 Pin) to HP adapter P1 port and connect HP adapter 8 pin to GPU. (Now it is converted from 6 to 8 pin).
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.