blubb
Have you already configured it in system settings?
Settings> System> Keyboard> On-screen keyboard> Keyboard> Appearance & Layouts> Custom input styles> plus icon> Language: "your language" Layout: Colemak> Add> confirm pop-up via "enable"
then
Settings> System> Keyboard> On-screen keyboard> Keyboard> Languages> Toggle on "your language" (Colemak)
If you have stored different languages/layouts, you can now switch between them using the world icon or by long press the space bar on the GrapheneOS standard keyboard.