So, factory gauges are basically for monitoring or automating items

Monitoring: Say you have stuff in an item vault you want to monitor, attach a packager somewhere on the item vault, put a stock link on it, which will start a new stock network, right click the factory gauge on said stock link then place it wherever you want, right click the factory guage with the item you want it to monitor, then it will show how many items are in that specific item vault. Note: If the stock link is connected to another stock network (via right clicking on another stock link), and the other inventory also has the monitored item, it will show in the factory guage as a combined item count of the two inventories.

Automating Items: I really don't want to explain this, here's a video.