I am using the new firmware version (6.0.3) and I got one issue using static route. If you already have any route to a specific GW and you try to add a new one to the same specific GW, the Fortigate shows the follow message.
"Gateway IP and interface conflict with Static Route #"
How to fix it?
Save your route using another GW IP. After that, right-click on the Route you want to change and click ">_Edit in CLI".
Run the command "set gateway 10.1.3.5", then type "end" and close the CLI Console.
Refresh your browser and you will see the result.
@Fortigate team, please provide an update to fix it up.