Some networking knowledge I figured out a good while back that should be standard, but I think it's poorly understood and worth sharing:

A route *without* a gateway address means you want the system to do an ARP (v4) or NDP (v6) lookup to find the MAC address of the *destination* and send/forward the packet to that MAC address.

A route *with* a gateway address means you want the system to do an ARP (v4) or NDP (v6) lookup to find the MAC address of the *gateway* and send/forward the packet to that MAC address.