NVIDIA Quadro RTX 6000 draws power from 1x 8-pin + 1x 6-pin power connectors.
Since the 1000 watts power supply already has 8 Pin (6+2) power cables, there is no need to use the HP adapter cable.
Connect G1 8 Pin and G2 6 Pin to the GPU as shown below:
It is not obligatory to use G1 to G4; you can use any available pins from G1, G2, G3, G4.
The below image shows a fully configured 1000 Watts power supply with Nvidia RTX6000 GPU and Video I/O installed on the HP Z4 G4 machine.
After installing GPU as described above, make sure to lock the PCIe bracket securely, which holds the PCIe card on the motherboard firmly. Make sure cables are not protruding in any of the fans inside the cabinet.