An ICA client that uses a protocol to communicate with the server requires that protocol to be installed on the server, and a listening port for that protocol to be created.
Note: A gateway is a MetaFrame Server to MetaFrame server connection that enables the display or advertisement of all published applications or MetaFrame servers on both subnets. The client is not involved in the creation of a gateway.
Note: Protocols installed on a MetaFrame server are installed through the Network Control Panel Applet just as they are in NT 4.0.
Note: Listening ports are created by default (2) when a specific connection is created in the Connection Configuration Application of the MetaFrame Toolbar.

Reference: MetaFrame Administration Module 6