|You have two options to set a pcs gateway.|
1. you can use two dhcp servers with reserved ip addresses. This is where you manually in the router associate a pcs mac address with a ip address. This way a pc you want using gateway1 doesn't get a ip from gateway2's dhcp server
2. you manually set the ip address, which includes the gateway, on each and every pc.
You still need to answer the question concering RDP wheither its outbound or inbound.
You seemed surprised by the answer you need to manually set the gateway.
Have you considered instead of two gateways paying for more bandwith for your existing one?