When moving workers carrying gas, be sure to right-click them on the new geyser so they will start returning from the new gas mine. If the gas mine is as close as possible to your headquarters then you don't need more than four workers collecting gas from that location. If you place any more on mining gas they will be wasted. If the gas mine is further away, you may need an additional worker or two to collect gas at the fastest speed. If you see more than one gas miner stacked up waiting for the gas mine to be free, you know you have too many mining it.
Gas is by far the most coveted resource.
It is used for upgrades and to build all the higher tech tree units and buildings. One should attempt to get multiple gas mines until gas is not a worry. Generally it is not a wise idea to specifically attack workers mining gas. Only 4 workers are usually mining the gas at a time, and it is really easy for your opponent to make another 4 workers if they are destroyed, then put them back on gas. Target the workers mining Minerals specifically because there will be many more workers in that area, and they will be fully exposed while they're harvesting.