Sharing the steps to run GUI application on target and verify from host for reference
You need to run below commands to keep X11VNC active on target
sudo service gdm3 stop
sudo service gdm stop
sudo -b X -ac -noreset
export DISPLAY=:0
sudo xrandr --fb 1920x1080
sudo x11vnc -geometry 1920x1080 -display :0 &
Once it is active, you can run any GUI sample. If you run an application in another terminal make sure you set export DISPLAY=:0 in that terminal before you launch application.
If VNC session is not active, you need to run above commands again to launch x11vnc
You can connect to target using tools like VNCViewer from host to view the GUI output