hello brother do u know what GNOME is-GNU Object Model Environment
so GNOME IS the default desktop environment in all distributions of GNU/Linux ,as its(GNU/Linux) the GNU project
Actually, GNOME is far from default. TWM is the default window manager for X11, the rest replace TWM. Also, it is only GNU/Linux if you're using GNU userland tools with Linux. In theory, you could use the Linux kernel without the GNU tools, it would just be troublsome to do so.
Other window managers that *could* be default:
All of which are GPL (as is KDE), the GNU Public License, but, that doesn't mean that any of them have to be used with GNU/Linux. For that matter, a distro doesn't even *have* to come with X11 on Linux.(Not to mention the non-GPL ones, CDE, 4DWM, JDE, etc)