the GMC-300Eplus has a "data port" which is a regular 3.5mm audio-out if I'm not mistaken. I understand that there are some apps for the Smart phone like "RadMeter" for example that can interpret these signals. What cable do I need for this connectivity? Is something that is regularly available or do I have make my own cable?
"Data port" is an euphemism. It is nothing but a plain vanilla audio connector, providing the audio "clicks", for which you need a plain vanilla audio cable.