We have a gdm config in the os-installer package (here) that, if I understand correctly, gets applied to the system in the postinstall step of the installation. (to replace the one that is applied to the live ISO in the infrastructure-tooling/image/solus-image-creator.py script)
This config differs from the one we ship directly with gdm (relevant patch here), notably in the WaylandEnable=false option, which is commented out in the former, but supposed to be active (i.e. disable Wayland) in the latter.
If I'm not mistaken this doesn't get overwritten by the actual gdm package, so systems are stuck with the config applied during the installation. At least on my system it seemed to be the case in my recent gdm testing.
I think we should as a first step make sure those configs have the same contents and in the future source the same config file for both purposes, so one doesn't have to change it in two places.