The gateway bridge is only for the data path between the packet forwarder and the chirpstack sever.
To send the decoded application level output of the chirpstack server to a particular broker, you have to modify the settings of chirpstack.
Do not change the gateway bridge settings - all you will accomplish by doing that is to make your gateway stop working. The only reason to change the gateway bridge settings is if you want to use an instance of chirpstack hosted in a different server.