@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.
Nice work so far @wint0178, great that you try expanding on the examples. Sorry for the late reply.
If you look at https://github.com/mysensors/MySensorsArduinoExamples/blob/bba998bce09bc5139eb4ca7a05b0279f4083ff88/examples/PressureSensor/PressureSensor.ino#L90 you see where the existing child ids are defines. You just add another one for the gas sensor, with a unique number (incrementing by 1 is an easy way).
Try that and report back on your result. If something is unclear, just post a followup.
For those of you who want an extremely simple build, I've reduced it to just 5 parts:
one 1-megaohm resistor (0805 SMD)
one Arduino Pro Mini
one Ra-01 LoRa module
Male header pins
Battery clips
See photos.
