There are some unclear things in your post, you say you have 4 gateways, 2 for cluster and 2 for HA, what do you mean by that, are you going to build 2 HA Clusters? Or are you going to build a Load Sharing cluster and 1 HA cluster?
On your network questions, your ping to the gateway fails due to the initial policy that does not allow any access to the gateway, except for the control connection from management. Therefore ping is dropped, but you should be able to SSH from management to the gateway.
When you say I'm unable to add the gateway in the Smartconsole, how are you trying to add the gateway?
The network is not needed to be directly connected, we manage FW's with management server in EU and gateway in Australia.
Regards, Maarten