There is nothing unique about the GW configuration to make them redundant, they have to have matching configuration as all control is done on the call server i.e CUCM. If using SIP for example you would have matching dial-peers on each GW and CUCM would route calls to redundant GW when needed.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.