The modules are automatically registered to a gateway the first time you connect/debug to it using mc-Studio. It will only talk to that gateway until you decided to connect/debug the same module on another gateway. If a module is registered to 2 or more gateways, the gateway with the best wireless integrity wins and will have the opportunity to transact sensitive data i.e. MQTT Publish/Subscribe, IFTTT etc.
Module Beacons on the other hand are passive, One way, Unencrypted transmission. Which means all of the gateways that are in range(registered or unregistered) will be able to receive the data and relay it to the IP Network through UDP messages.