No, there is no need to change /etc/sysconfig/desktop, even when it existed! If you are using GDM, it already supports XDMCP, so there is no need to change anything. (Many other desktop managers also support it, you do not necessarily need to use GDM). But, you should most probably disable running GDM with Wayland. For which, you'd need to disable wayland in /etc/gdm/custom.conf
by uncommenting the following line:
WaylandEnable=false
To enable XDMCP with GDM, KDM, and some others, you can use the great Arch wiki:
https://wiki.archlinux.org/index.php/...
BTW, don't forget to open the corresponding port in firewall too. You might use the graphical firewall configuration utility or use firewall-cmd
to do that.