A tag is an object, so it can be everything you need. In your case it will probabily be a string representing the Gauge ID.
When an MQTT message arrives, containing a valid ID, you loop on the CLV items to find the one having that string as its Tag.
That's why I suggested the "trick". Looping can take time. Even a short 10 gauges list, if you have to update gauge #9 it implies that you traverse the CLV from start to that position in order to find the corresponding ID.
Using a map, a list or any other collection (even an array of longs) to maintain the position of any specific ID in respect to the CLV item index, let you discover very quickly the item you hace to update. You construct the list in parallel with adding panels to the CLV.
This begins to be useful if you have a lot of gauges or if you have to update them very frequently.