GNOME is a desktop and was started in 15 August 1997 (some GNU folks didn't like that Qt was owned by a company, thus saw KDE (which came from CDE) as a risk so created their own desktop instead of the existing ones like CDE, KDE, XFCE etc).
X windows however is not a desktop, but is much older starting in 1984 (version 11 is from 1987).
GNOME Itself will run on X windows, or will run on Wayland (therefore X windows is not used at all; Wayland having replaced X windows in this example). Likewise you can still use X windows and not GNOME, ie. a different desktop like KDE, XFCE, LXQt and many more.
X provides the basic framework for a GUI environment: drawing and
moving windows on the display device and interacting with a mouse and
keyboard. X does not mandate the user interface – this is handled by
individual programs. As such, the visual styling of X-based
environments varies greatly; different programs may present radically
They are different things (running at different heights of your software stack), so differences to me makes no sense - differences will far outnumber similarities.