Here’s a solution on how to use a different keyboard layout in GDM (different then the default en). The problem was that if you wanted to use any special character (that i.e. needs alt gr) or any other character specific to your keyboard layout (in my example čćžšđ), you couldn’t do it even though you already set locales globaly on your machine. I did this on arch distro but my guess is that it will work on any other too.
Execute the following in your terminal as root:
and then replace “en” layout with the one you need (in my example hr):
<merge key="input.xkb.layout" type="string">en</merge>