I have looked into using udev. I found out that the Arduinos with FTDI interface do have unique serial number whereas the cheaper clones with CH341 interface do not have serial number so I have ordered nanos with FTDI chip to continue my experiment.

I have also managed to get the gateway to send out sensor's data with a gateway id other than 0 by making one line change in core/MySensorCore.cpp:
if (message.destination == _nc.nodeId) -> if (message.destination == 0)

Assuming that this hack does not create other problems, I can now have multiple gateways with different ids. But I am facing a new hurdle: mysensors plugin for pimatic (controller software) is designed to support only 1 gateway! I can probably make it work by creating individual plugin for individual USB port but I am not so sure that creating a system with unsupported hacks is such a good idea. So I will pursue another approach of creating a controller with a bunch of sensor nodes hanging off USB ports.

I am using Raspberry Pi running Pimatic as controller and nano as sensors. I have several type of different sensors and I would like them to be USB plug-and-play. However /dev/ttyUSB gets assigned randomly so unless the nano has some kind of ID embedded, the controller has no way to associate a certain /dev/ttyUSB to a certain sensor. I am not familiar with MQTT but will start looking into it.

Just got on board, great stuff! I was able to get DHT11 to send data to the controller by commenting out #define MY_RADIO_NRF24 and add #define MY_GATEWAY_SERIAL to HumiditySensor.ino in the example sketch. In my application, I need to have several gateways, each with its own type of sensors. I would like to know if there is a solution. I poked around and found #define GATEWAY_ADDRESS ((uint8_t)0) in core/MySensorCore.h. If I changed GATEWAT_ADDRESS to any other value besides 0, gateway will not send out sensor's data.