The Gateway is both a Channel Access server and Channel Access client. It provides a means for many clients to access a process variable, while making only one connection to the server that owns the process variable.
It also provides additional access security beyond that on the server. It thus protects critical servers while providing possibly restricted access to needed process variables.
The Gateway typically runs on a machine with multiple network cards, and the clients and the server may be on different subnets.
The CA Gateway CI is hosted on CloudBees.
More details are available on the CA Gateway main web page.