You need to set the ConnectPolicy key on the server. This will let you list all the available desktop types on that host (e.g., GNOME and KDE):
ConnectPolicy autocreate=0,autoconnect=0,desktop=1
From the article:
How to configure NoMachine 4 on Linux to restore some behavior typical of NX Server 3.5.0
https://www.nomachine.com/AR07K00681