Ok will answer my own question then:

function changeRGBGatewayColor(rgbColor, brightness)
gatewayName = '<NAME OF YOUR XIAOMI RGB GATEWAY>'
gatewayIDX = otherdevices_idx[gatewayName]
url = 'http://<YOUR DOMOTICZ IP ADDRESS>:<YOUR DOMOTICZ PORT NUMBBER>/json.htm?type=command&param ...

I've a Domoticz version 4.9700 running and a Xiaomi RGB Gateway on firmware 1.4.1_157.0143.

What I did found out is that you can control the brightness of build-in RGB light with:
commandArray['Xiaomi RGB Gateway']='Set Level:'..tostring(10) -

You can turn on/off the light with:
commandArray ...