Uhm, I calculate it and I get about 4100 rpm at maximum speed.
If can help, SpeedFan reports about 4110 rpm for CPU fan.
Anyway, if I disable "EC Support" in HWiNFO64 Settings only GPU fan will disappear, CPU fan speed is still there, under "ACPI: ASUS G751JY".
Then, if I disable "Evaluate...