Target is the destination after the netmask is applied (basically use your NTP server IP, and netmask 255.255.255.255) interface is the one used to send the packet, gateway is the router to send to, metric is a "cost" lower is the route that's preferred. Doesn't matter here unless you have more than one route to this specific IP.
Of course it's not easy to choose a gateway if your device moves around and gets DHCP addresses. In that case you probably need hotplug scripts or some such thing.