Toby, here's my 2 cents.

- If you have a friendly DNS entry (for your users), and it points at one of your server IP's, then the gateway on this server is being used.

- Multiple gateways I believe are only necessary when you have an upstream load balancer that receives the http traffic and then it can divvy up the work to multiple gateways.  If one of the gateways goes down (or the server that it lives on), then the load balancer can be configured to not send traffic to there.  In the case of this, the DNS points at the load balancer.

- I've in the past dug into the logs, and have observed that Tableau internal processes (i.e. vizql) direct traffic to the gateway / Apache Tomcat and it starts with the primary (if a gateway is here), followed by each of the workers in sequence until it finds a valid gateway.  For us, we don't run anything on the primary besides the required licensing and backup and scripts.  So we determined, it didn't make much sense for worker1 to direct traffic to the primary only to then be redirected to another process on worker1 (i.e. dataserver).  So we chose not to run a gateway on primary.  The savings was negligible, but it was more a principle thing.

P.S. did tabcmdadv help?