I was just working to get some OIDs myself for routing, sorry it's probably late, but:
At least on my FortiGate 30E the OID for ipRouteTable is:
See attached image, I've highlighted what I used to trigger my Operation's Manager alert. (If NextHop changes - then alert). The OID is .22.214.171.124.126.96.36.199.1.7 and then you append the IP to the route, for instance for last resort: .188.8.131.52.184.108.40.206.220.127.116.11.0.0 - the value shall be the NextHop IP according to the route table.
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.