Gnome and KDE are desktop managers. These are the applications that actually give you the graphic interface between you and the Linux operating system. There are other desktop managers out there...
That is a matter of user preference, and not everyone will agree with either the verdict or the criteria for choosing.
Yes. Both are fine.
GnomeThere is a KDE version named Kubuntu, however
Ubuntu uses GNOME by default. However, a KDE version is available, named Kubuntu. It doesn't have all the bells and whistles of its GNOME counterpart