Gas smokers are easy to use and though they don't generally provide the kinds of advanced controls that Electric Smokers do, they are reliable. Gas is also a faster and more responsive fuel than electricity, though in many ways gas and electric smokers work very similarly. With a gas smoker, a small burner heats a pan of wood chips and typically a water pan for moisture to produce heat, smoke, and a moist cooking environment. The burner can produce more heat than a heating element, so gas smokers can provide better smoke. On the other hand, the combustion itself doesn't produce smoke so these units won't make the kinds of authentic smoke that a Charcoal Smoker can. Gas smokers are generally reliable though so there isn't a need to tend to them and they can be loaded up and started in minutes without having to worry about building a real fire.
Broil King Vertical Propane Smoker
Broil King has been making some of the best gas grills on the market for decades. Recently they entered into the vertical smoker market with models like this one. The price is higher than the competition, but this is a well built gas smoker than has a good sized capacity and is simple to use. Probably the biggest standout feature of this smoker is that among the competition, it is the only one produced in North America.
Smoke Hollow 40-Inch Propane Smoker
Like most of the gas smokers on this list, the Smoke Hollow 40-Inch is a big block of a smoker. It really is simple and it costs around $250USD, but it has great capacity. It comes with four cooking grates that actually gives you the space to do four whole briskets or many racks of ribs. Many gas smokers of this type can't hold a whole rack of ribs without cutting them in half or at least trim them down. This one can. It is simple and no, not built all that well. But cared for, covered, and kept clean it will reliably produce smoked foods for several years. It is also easy to use.