This article explains how to define a secondary IP address for the interface and use that address as the local VPN gateway address.
The IP address of a VPN gateway is usually the IP address of the network interface that connects to the Internet.
The benefit of the option stated here above is that the existing setup is not affected by the VPN settings.
IPsec, VPN, Phase1, FortiOS, Site-to-Site VPN, tunnel, secondary IP, peer.
To add the IP address:
As of Forti OS 6.4.9 -- ID 728468: 'Local-gw' IP address must be assigned to the interface to work properly, either as a primary or secondary address.