Fortinet is just an IPSEC VPN server - you don't specifically need their client to connect to it. The IPSEC HOWTO details a list of various options you have for setting up a Linux VPN client. There are also a few commercial linux IPSec clients such as Shrewsoft.
Setup a Site to Site IPSec VPN with Strongswan on Ubuntu Networking VPN Linux IPsec Today we will setup a Site to Site ipsec VPN with Strongswan, which will be configured with PreShared Key Authentication. After our tunnels are established, we will be able to reach the private ips over the vpn tunnels. Setting up a VPN with your iPhone using L2TP, IPSec and Linux Dec 11, 2011 Learn how to Create Your Personal IPsec VPN Server in Apr 28, 2020 Best Open Source VPN For 2020 - 5 Choices To Consider
IPsec VPN on linux ? | Fortinet Technical Discussion Forums
VPN – MX Linux Jul 01, 2016 IPsec
How to Connect to L2TP/IPsec VPN on Windows
client – This allows you to set a username to go with the CHAP secret. Here, we use the wildcard * to allow any username. server – The name of the server. We specified this in /etc/xl2tpd/xl2tpd.conf above. secret – The CHAP secret itself. Again, this is a password and should follow good practice