How do I add a level gauge for 100 lb propane tank?

Put it on a scale.

The only thing that a pressure gauge tells you about a propane tank is its temperature, or that it's completely empty. The tank should have a "tare weight" marked on it (the empty weight) and if it weighs 100 lbs more than that, it's full.

Propane is transported as a liquified gas under pressure. So long as there is any liquid in the tank, pressure indicates tank temperature.

Weight will tell you how much has been used.

The traditional solution is TWO tanks, an indicator, and a valve that switches which tank is "primary" (and raises the indicator when it becomes empty) and which tank is "reserve" (which the valve draws from when the primary is empty.) You remove and refill the empty tank, and switch the valve (which causes the indicator to lower) to make the other tank "primary" until it's empty and you repeat.

Much larger fixed tanks typically have an internal float gauge to sense the liquid level, which is not terribly accurate, but generally good enough to tell you to call the truck and get it filled up before you run out. I've never seen one for small tanks.