"Wayland" is also an implementation. The protocol and implementation share a name. By the look of things, Gnome does indeed require it (that is, various "libwayland" packages) on Ubuntu.
Wikipedia:
> Wayland is a communication protocol that specifies the communication between a display server and its clients, as well as a C library implementation of that protocol.
That's the C library also named Wayland and made by the people guiding the Wayland spec on which Gnome depends, no? Or are there three things going by the name Wayland, one of them not actually associated with the Wayland project?
AFAIUI, e.g. KDE doesn't actually use any of that. (I could be wrong, I honestly don't care that much about the minutiae.)