I recently switched to Gnome on all my machines replacing KDE. One problem that immediately bothered me was that the "eu" layout is not in the list of available keyboard layouts in Gnome settings. Using
setxkbmap eu was possible, however, Gnome would reset the layout every few minutes which is very annoying.
Using the gnome-tweaks tool to enable additional input sources did not work for me.
After some research I finally stumbled across a solution that works for me (and survives reboots!):
gsettings set org.gnome.desktop.input-sources sources "[('xkb', 'eu')]"