I use mostly KDE on Kubuntu, so I'm not completely familiar with Gnome. But I know that "vanilla" Ubuntu user interface / desktop environment is called Unity and is based on Gnome but is not the same as Gnome. There's pure Gnome flavor available, see http://ubuntugnome.org/documentation/faq/
. Also the KDE environment is somewhat modified from the original KDE. You can mix different desktop environments on ubuntu -- Kubuntu, Xubuntu etc. are really just software packages and configuration on top of ubuntu base. You can e.g. install Ubuntu and afterwards install kubuntu-desktop package on it, and it becomes Kubuntu.
But Fedora and some other distributions are different. You can't mix ubuntu-based distros and Fedora. They have different infrastructure and packaging systems. Fedora has its own flavors of Gnome and KDE. I'm not nowadays familiar with Fedora, so I can't say how much its Gnome and KDE differ from vanilla Gnome and KDE.
Using polytonic Greek on Kubuntu is easy. In System Setting -> Hardware -> Keyboard -> Layouts add a layout. Select "Show layout indicator". The indicator is in the Panel (the KDE bar). Click it to change the layout, or use configurable shortcuts. I don't remember anymore but I believe the Greek layout is the same as in Gnome or plain X.