"gateway" can be many things.
In your case, "gateway" will push calls though an extension.
I cannot tell you how to do this on the "alcatel-lucent pbx", but I can tell you how to do this on FusionPBX.
1. Create the extension on the destination PBX
2. Go to FusionPBX Accounts > Gateways, and fill in (at least) the Username, Password, Proxy settings
3. Go to Dialplan > Outbound Routes and create a route that will use the new gateway created
You should be able to call through to extensions on the other PBX this way though the extension on the destination PBX (not back without replicating this on the other PBX though)