One of the main design principles of the WIZ is that while it can run stand-alone, it is designed to run in an environment with many many other WIZes nearby.

Thus it is paramount to have a very fast method for WIZ-to-WIZ communication. This is the  "gateway" circuit. A gateway register is one which appears directly on the buses of two cores at once. What one writes to it, the other reads, and vice versa.

It is not a pair of registers, one on each WIZ, which connect to each other, but is instead a single register wired directly to both WIZ busses.

Each core sees it as if it were its own register, connected to its own bus. Each is "unaware" of what is on the "other side", or "backend", of the register.

Indeed, what we have is that one WIZ is the backend device of the other, and vice versa.