There is no "GNOME on Wayland" - Mutter <b>is</b> the window server - it talks directly to the Direct Render Manager and manages buffers. It's just that most of the GPU interfaces are standardized in mesa and the kernel so there is no need to reinvent the wheel and makes it so Wayland window servers + window managers are feasible to implement, and also why wlroots exists.

You could have implemented Mutter on Windows NT using DirectX and such - Microsoft has done such a thing in WSL2.