I created an article how to install OpenVPN client in CLI text model HERE. Now it is a time to show how to install OpenVPN in a GUI interface.
sudo apt-get update
sudo apt-get upgrade
sudo apt install network-manager network-manager-gnome openvpn
sudo openvpn-systemd-resolved network-manager-openvpn
sudo network-manager-openvpn-gnome
sudo apt install network-manager-openvpn-gnome
restart pi
Sources:
https://www.vpnunlimited.com/help/manuals/raspberry-pi-openvpn-setup
https://askubuntu.com/questions/913866/vpn-plugins-are-not-installed