If you mean the MySensors specific node with ID=00 called Gateway (and not bridge )...
The RaspberryPi don't need a Gateway, that is: no Arduino is required, the MySensors protocol is managed by the Controler itself. A nRF24L01+ module on a specific adaptor that make the interconnexion is enough.
Look at the
written by EasyIoT for more explanations.
I think he means a pcb board with dual 5v 3.3v supplies so thst he can run his 5v pro-mini and the NRF. Check the mysensors site, they show a drawing where you just add a 3v3 regulator to the line for the radio.