The Gauge Dropper is an important tool used to interact with machines that work with fluids and/or gasses, such as the Chemical Infuser.
If a Mekanism machine has a fluid and/or gas gauge, then you can use the Gauge Dropper on it. Open the machine's GUI and pick up the gauge dropper as you would if you where moving it to a different inventory slot. Hover over the gauge you wish to interact with, and then do one of the following:
Left Click: Will collect up to 1000mB of the gas/fluid and store it in the dropper Right Click: Will deposit whatever is in the dropper into the tank, if it is compatible and is of the same fluid/gas type (you can't put oxygen in a tank that has hydrogen in it, for example). Shift + Left Click: Will dump the entire tank.
You can also dump the contents of the gauge dropper by selecting it in the hotbar (outside of any machinery GUI) and use Shift + Right Click on any block. The contents will be destroyed in the process.