You can create a cluster of all Gateway Appliances or of all Gateway Services.
Creating a cluster of Access Gateways of the same type ensures that the user experience is always the same, regardless of which Access Gateway the user establishes a connection to. For a list of the differences between Access Gateway Appliance and Access Gateway Service, see Feature Comparison of Different Types of Access Gateways in the NetIQ Access Manager CE 24.2 (v5.1) Installation and Upgrade Guide.