A propane tank gauge is a requirement but some propane tanks do not have it. It is used to easily tell the level of when you need to refill the tank. In case you do not have the gauge, you will need to check the level of propane by yourself.
From my knowledge, no you don't need a gauge. But everything you use with propane should have a regulator. That causes the correct air mixture. Also they have updated the tanks a few years ago. If you take it in to a local filling station and try to have it filled, the machine will not fill your tank if you have a outdated tank. Most places will swap your old tank for a new one if that's what you want to do.