Depending on exactly how you want things to work, and what version of FortiOS you're using...
If you want the 100Mbps link to be used only if the 1000Mbps link fails:
Set the distance value on the route to the gateway on your 100Mbps link to be a higher number than the distance value on the route to the gateway on the 1000Mbps link.
Set the distance value the same on both routes and adjust the priority field on the 100Mbps link to be a higher value than the priority on the route to the 1000Mbps gateway.
If you want to be able to sometimes use the 100Mbps link for selected traffic e.g. guest WiFi access, whilst letting everything else go out the 1000Mbps link, you will need to configure the distance and priority values on both routes to be the same, then use a policy route to selectively route the WiFi traffic out the 100Mbps link interface.
Make sure you have dead gateway detection enabled.
From trial and error: using 2 routes with same distance with higher priority on the secondary link results in slight loss when failing over, and no loss at all when failing back. When using higher distance, same priority on the secondary link, there was slight loss failing over, and slight loss again when failing back.
Best of luck,