Hi Neeeco,
I experienced the same problem, with having multiple instances of the custom html component.
From what I understand, the problem/conflict appears because the gauge Class Name or ID is hardcoded. When having multiple instances of the html components, all gauges will have the same style/behaviour because Class name or ID is identical.
I hope this make sense 
Maybe someone from the anvil team have an idea how to solve the above.
If I were you I would not use the custom html component and instead create the gauge directly in the python code (form load).
This can be achieved using anvil.js and anvil.js.window where you can e.g. just drag and drop a spacer on to the form and via python code, replace the spacer content with your Gauge object. In this scenario you can have multiple instances.
Please see the above example. I hope you find it useful