The keyboard mapping stored per-user in .dmrc only contains layout and variant, separated by a tabulation. It doesn’t store XKB options.

The fact that GDM stores layouts per-user even when the user doesn’t change the layout makes this worse. Selecting options in the Xorg configuration is useless since gdm will drop them at login time anyway.

Fixing this requires changing the semantics of the Layout key in the .dmrc file. Since it is shared with kdm and other DMs, and not specified anywhere, there could be an impact to change it, so it might not be fixable without synchronizing with them.