Assuming that drivers are working etc. (which is reasonable since Gnome works), there are two ways in KDE to start a program on the discrete GPU:
- open a terminal and issue, for instance,
switcherooctl firefox - right click on the app icon, then “Edit Application…”, in the “Advanced” tab check “Run using dedicated graphics card” then save; from now on the app should start using the discrete GPU.
That assumes that you can at least login with SDDM to the Plasma desktop, as it does here, but I have an “Optimus” setup with Intel + Nvidia and that might make a difference.