Cannot switch keyboard layout at GDM login

Go to Settings -> Region & Language. In the upper right corner of the window you should see a Login Screen button. If you click that button, the UI switches to a variant for system wide settings. From there you can add the input sources you want to be able to use on the login screen. If you have more than one source selected, an input source indicator will be shown on the login screen after next reboot.

The button may only be visible if more than one user is registered on the system. As a workaround you can create a new user and delete it after you made the changes you desired.

In my Ubuntu Budgie 18.04 I just run on terminal:

sudo nano /etc/default/keyboard

and then change the unwanted layout at xxx in the line:

XKBVARIANT="XXX"

save and then reboot.

P.S.: just copy the layout "yyy" from the first line to the XKBVARIANT line

XKBLAYOUT="yyy"