I’m not sure if this will work, but what you can try is replacing the %desktop-services with this:

(modify-services %desktop-services
  (gdm-service-type config =>
                                   (gdm-configuration
                                     (inherit config)
                                     (wayland? #f))))

I’m not sure if wayland is actually the problem though, especially if gdm was working fine before you swapped to lxqt. Another thing i’m thinking of is maybe lxqt isn’t properly exposing a session file to gdm, and perhaps gdm crashes because it doesn’t find any sessions to start. I think i’ve seen other people adding this session file manually but i would have to look up how it’s done. The thing is i don’t use a DE myself, and i’m using greetd+gtkgreet as a display manager and it just launches me into a tty, and then my bash_profile has a snippet that checks which tty i’m and whether it’s empty, and if it is it launches my wayland compositor of choice.