Interestingly, the PCIe 8-pin power cable into a GPU doesn't carry all of the return current. If you put a current clamp meter around the +12V wires and then the ground wires, you'll measure more amps on the +12V wires than the ground wires. This means some of the return current goes through the PCIe slot into motherboard and makes its way back to the PSU. This lets the GPU create audio noise because GPUs draw high current pulses at the frame rate of your monitor, which means the return current through the motherboard has high current pulses, which can create ground bounce on the motherboard where the ground voltage level moves up and down and that can affect other devices in the system.

I don't totally know how that noise would traveling over the ground shield of the HDMI cable into the analog section of the Denon receiver though. Maybe some of that GPU return current is going through the HDMI cable, through the Denon receiver to mains earth, and then through your building wiring back to the ATX PSU? Grounding is freaking weird.