Does anyone know where in the index.html file is the section for adding an additional node Type?
Stock node types are defined in metrics.js (https://github.com/LowPowerLab/RaspberryPi-Gateway/blob/master/metrics.js)
I recommend you add your custom node and metrics in the userMetrics (https://github.com/LowPowerLab/RaspberryPi-Gateway/tree/master/userMetrics) folder, following the example there.
When you make edits to metrics you must restart the Gateway software for changes to take effect.