The BEC Gateway works as a proxy that sits between Bryant Evolution Connex thermostats and their cloud service. It monitors the traffic between the two and allow clients to learn about the state of the thermostat and in the future will allow them to modify that state as well.
The BEC Gateway project was created to facilitate the integration of these thermostats into Home Assistant. It is the the main piece behind the BEC Home Assistant Add-on. The add-on works together with the BEC Home Assistant Integration to complete the BEC Home Assistant support.