What did you name your nodes in your Couchbase instance? If you look at the "Server Nodes" tab, this is one of your nodes with the name "192.168.2.102". I see that from your gateway gateway computer you are trying to reach the Couchbase cluster using the address 75.76.221.21. You checked the network connection from your gateway gateway, try "telnet 192.168.2.102 8092" and see if it connects.
Another thing, most often run the synchronization gateway, redirecting to the configuration file, you can find them in the examples directory. For example, "sync_gateway <examples / config-server.json"
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