Before we talk about how to make smoke on a gas grill, it needs to be pointed out that some gas grills have dedicated smoker boxes. If yours does, then please read your grill manual. Otherwise, continue.
A gas grill works by pumping air through the cooking chamber. The burners heat air, drawing it up through the bottom of the grill, where it reaches the food and then the lid, before exiting out the back. Gas grills are not terribly adept at retaining smoke. However, it is possible to add some light smoke flavor to foods, mainly if the cooking times are long. The secret is to produce smoke over a long period of time. This is when low, and slow cooking becomes challenging, though it can be done.
About the Wood
When making smoke, use large wood chunks on charcoal or fine wood chips in electric or gas smokers. Of course, pellet grills are fueled by wood pellets that produce smoke. In a gas grill, we need a smoldering fire that provides little to no flame but a lot of smoke. That is the real secret, if the wood you are using for smoke starts burning like a real fire, it won't produce much smoke, and it will burn away quickly.
Wet vs. Dry
For years we were advised to soak our wood chips and chunks in water before adding them to the fire. It was thought to help them smolder more and produce better smoke. It isn't true. Large chunks don't absorb much water and wet wood chips steam until they dry out. Soaking your wood only delays smoke production.
The easiest and cheapest way to get smoke into your grill is with an aluminum foil smoke bomb. Make a pouch out of foil, fill it loosely with wood chips or pellets. Do not pack it tightly. There must be airflow so that the wood can receive the oxygen it needs to smolder. With a knife, punch several holes all over the pouch. Doing this lets air in and the smoke out. For long cooks, it is best to make several of these and replace them as the smoke runs out.
A better way to make a smoke bomb is with a disposable aluminum pan. Find one that fits your space. Add an inch or wood chips to the bottom. Cover with a sheet of aluminum foil and poke holes through the foil. Now place that pan over a burner that you will be using during the cooking time. It might be necessary to turn that burner to high to get smoke production started. After that, you can lower the temperature.
Smoke Making Devices
Homemade Smoke Bombs can be unpredictable and may not give you the best results. There are, however, several accessories you can buy that will produce smoke in your gas grill. Some work, many do not. I have picked from the best to give you better smoke making capabilities.
A-Maze-N 12 inch Pellet Tube
The A-Maze-N Pellet tube is a stainless steel mesh device, open on each end. It is filled with wood pellets. You get to choose the flavor you want. Place the tube inside the grill and light one end. You will need a long lighter to get it started, but once the pellets are burning, that's it. The tube design keeps the pellets burning slowly. This 12-inch unit will produce smoke for an extended period. Since this doesn't need the heat of the grill's burners to combust the wood, you can place this tube anywhere inside the grill. The downside of this device is that it burns wood pellets, and it is hard to find them in small bags.
Buy the A-Maze-N 12 inch Pellet Tube
Char-Broil Cast Iron Smoker Box
The advantage of this smoke box from Char-Broil is that it is cheap. You can get one for about $10. It is solid, and will hold a lot of heat. Plus it can be fitted below the cooking grate if you prefer. Of course, if you do that, it will be hard to refill. I do suggest that if you are using cast iron, be careful around the metal components of your grill. Cast iron can scratch metal surfaces, opening them up to corrosion.
Buy the Char-Broil Cast Iron Smoker Box
Charcoal Companion Stainless Steel V-Shape Smoker Box
This model fits between the flame tamers of many models of gas grills (though more specifically Weber Gas Grills). This puts it close to the fire so that smoke production is good. It has a hinged lid so you can easily add more wood, except that you have to remove a cooking grate to get to it. For shorter cooks, this is a good model. It has a stainless steel construction that you can run through your dishwasher with ease. Check the design of the flame tamers of your grill before buying this one.