sensor id is not the same as node id.
sensor id = 255 is used for internal stuff like id request and battery level for node. You can assign 0-254 to your own attached sensors (to the arduino node).
@jocke4u I run 2 RF gateways and it's not a problem. When you 'migrate' a node to a new gateway make sure to run the mysensors clear eeprom sketch first to get rid of the old routing table or you might have issues.
@A3V such a feature would definitely make sense now that gateways can have local sensors. But no-one has added it unfortunately.
A solution could be to use the code at https://github.com/mysensors/MySensors/blob/2e00bf6a10f76d6aaa1999e12313237bc3edabd3/core/MyTransport.cpp#L446 as inspiration and do something similar at https://github.com/mysensors/MySensors/blob/2e00bf6a10f76d6aaa1999e12313237bc3edabd3/core/MyGatewayTransportEthernet.cpp#L175
