For Gnome use YaST > Software > Software Manager. Choose Patterns from the tabs above and look for the Gnome desktop ones. Select and install.
At Gui login (so do not use auto login) you can choose which DE you want to use for the moment.
And I suppose thet different desktops might have different ideas about which applications to use for which files types. Seems logical to me that Gnome will use another editor than KDE when you click a text file. And of course a lot of this is configurable.