First of all, I haven't ever seen a Windows workstation route successfully to the Internet with an incorrect gateway. Yes, traffic will still flow to local resources on the same LAN, but after that, the workstation is dead in the water. The Fortigate will work the same way. All it needs to know is where the next hop is to send any traffic not local to itself. If you (could) specify a network off of it's attached interfaces, it will have no clue where to send the default traffic because those remote networks will be unknown to it. Traffic will just die. Grab a basic networking book and browse through it. The Internet is based on next hop, not two or three gateways away. If everyone knows who their peer is to send the unknown traffic, then Internet life is good.
The way your ISP provides the public subnets are very common, which we do to our customers too. Technically the /30 subnet is just for the interface between your device (FG) and ISP's router (126.96.36.199 for ISP side, 188.8.131.52 for FG WAN IP). Then the ISP route is configured to deliver /29 subnet toward 184.108.40.206. So if the device that has 220.127.116.11 is a FW like FG, you can use the /29 as a routable subnet by assigning to inside interface of the FW (or you might want to split it to two /30s for two different interfaces) or you can break it up to 8 /32s and use each in VIPs.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.