The core clock/clock speed/clock frequency of a graphics card varies depending on the load presented to the graphics card.
When running idle it will be very low, only rising as the graphics card is required to do more processing. As the load reduces the clock speed will fall back to lower levels.
A CPU functions in much the same manner. You can improve the the way the CPU and GPU work in macOS by using power management that is suitable for your hardware to finesse the behaviour of the CPU and GPU when using pc hardware to run macOS.