OpenFortiGUI

Install FortiClientSSL VPN client
wget -q http://www.unibamberg.de/fileadmin/rz/vpn/Linux/VPN_Linux_FortiClient.tar.gz -O /tmp/VPN_Linux_FortiClient.tar.gz
sudo tar xzf /tmp/VPN_Linux_FortiClient.tar.gz -C /opt/
sudo /opt/forticlientsslvpn/fortisslvpn.sh

Install openfortigui
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 2FAB19E7CCB7F415
echo "deb http://styrion.at/apt/ ./" | sudo tee /etc/apt/sources.list.d/styrion.list
sudo apt-get update
sudo apt-get install openfortigui

Compile
https://hadler.me/linux/openfortigui/
apt-get install -y build-essential qt5-default libssl-dev
git clone https://github.com/theinvisible/openfortigui.git /tmp/openfortigui
cd /tmp/openfortigui && git submodule init && git submodule update
cd qtinyaes && git submodule init && git submodule update
cd .. && qmake && make -j4
sudo cp openfortigui /usr/bin/openfortigui

Links
https://styrion.at/apt/
https://github.com/theinvisible/openfortigui
https://www.uni-bamberg.de/fileadmin/rz/vpn/Linux/VPN_Linux_FortiClient.pdf
https://hadler.me/linux/openfortigui/