The basic idea is that two or more gateways share a virtual IP on each subnet they are connected to.
The gateways also share information about connections between them.
This enables one gateway to take over for the other in the event of a failure.
There are two basic methods to provide the virtual IP: ClusterXL and VRRP.
You configure each individual gateway in SmartConsole and then create a cluster object that includes those gateways.
Refer to the following for additional information: R80.10 ClusterXL Administration Guide