The gauge on the right gives unregulated tank. It basically tells you how much gas is left in the tank. The one on the left gives regulated pressure to the torch. This actually controls shield gas flow. I usually adjust mine in the range of 7-10 lbs with the trigger pulled and gas flowing. It may take a bit of trial and error to find what is right for your setup plus adjust for conditions as mentioned above. The owners manual may give some guidance.

If you get splattery porous welds, that's an indication you need more gas flow.