If you just want to use the german umlauts with an US keyboard, XKB already includes a layout for that. Gnome 3 just doesn’t show it by
default. To get Gnome 3 to show it, install the
gnome-tweak-tool, navigate to “Keyboard & Mouse” and enable the first checkbox,
“Show extended input sources”. Now the “Region & Language” dialog shows the “German (US, with German letters)” layout.
To integrate a completely custom keyboard in Gnome 3, check the Cinnamon post, it works the same way in Gnome 3.