No. Having multiple Gateways does not mean that your Tableau Cluster has high availability, and will automatically 'failover'
If a Gateway process fails (or that server fails), and your end users are pointing at that server, then they will no longer be able to connect to the tableau cluster, until they are pointed at another server running Gateway process. The other Gateway servers will then be able to handle the workload.
This is the function of an external Load Balancer, to ensure that the traffic is pushed to an available server.
Hope this helps