Fortigate 6.0.3 has a bug that will give you the “Gateway IP and interface conflict with Static Route 1” This is a bug.

Work around:
Upgrade to 6.0.4 or add the route via the CLI.
config router static
edit 3
set gateway 10.10.80.1
set device “port2”
set dstaddr “RFC1918-10.0”
next