Couchbase views and GSI

I’m starting discovering couchbase GSI and i’m still having some ambiguities about views and GSI.
In fact, all the documents I read about views are about the scatter-gather operations that happen between the available nodes to collect data from all the nodes ,combine the data and send the response back to the client.
But isn’t the replication architecture of couchbase and specially the DCP protocole ensuring that all nodes having the same data?

Another question, is it possible to use “JOIN” when creating mapreduce views?