OK... when you use RubyAMF you're using Flex RemoteObjects (unless you're not using Flex) to make calls to Rails controllers. When you setup your Flex project you had to create and configure a services-config.xml that contained a service destination you probably named (id) "rubyamf." It's configured to use an AMFChannel with an endpoint uri that directs all rubyamf RemoteObject calls to the RubyamfController's #gateway action that does its magic and invokes the controller and method you specified in you request.
So in short... RemoteObject makes POSTs to /rubyamf/gateway
Unless you're sharing your controllers with other clients/presentations, you don't even need to map/route any resources besides the rubyamf/gateway that works with the default route.
I hope that helps.