There is no real manual that I have been able to find on your Gateway. Dest. IP is going to be the Router IP. The Source IP is the device you want to limit. Just take the IP that is assigned in the Gateway to that device, and plug it into the Source IP, along with the netmask that the gateway uses for devices connected to it.
The problem is that every gateway software is written different from the other. It may not work properly unless you enter all of the values. You need to find out what the gateway uses for Priority values. Again, without a manual for that gateway. There is no way to answer all of your questions.