the wifi setup portal, when you connect your smartphone or
in platformio.ini or
in user_config.h
WIth MQTT you can listen to all the topic of one level by replacing its name by a +
You can let it like that without any issue
See it here
Note that depending on the distance between gateways and your proper network the deduplication may not work (if the signal received is faster compared to the communication between gateways)
The best way when you subscribe would be to do it to rfbridge/+ ; this way you will receive all the gateways messages even if they have different names.