You can use a smoker box as the poster above suggested.
You can cook the food with some smoke present, but it's not the same as "smoking" meat.
"Smokers" for true smoking are very slow cookers. They're usually made of something that will retain heat (such as ceramic), or use a water pan and a mix of steam and smoke to cook. You can somewhat duplicate this with a water pan and a large kettle grill, but you will have a hard time doing this with a gas grill.
You can "cook" with a gas grill and smoker box, or with a standard kettle and wood chips, and get some smoky flavor on the outside of the meat, though.
It's best to soak the wood chips for at least several hours, though, as you don't want them bursting into flames and coating your food with soot.
Gas grills are great for the convenience, or for cooking delicate foods where precise temperature control is needed, like vegetables. It's hard to duplicate the results you can get with charcoal grills on a gas grill with meats, though.
We have a gas grill that rarely gets used, a large webber kettle, and a smoker. I want to build a larger ceramic smoker and ceramic or brick oven too, one of these days.