Well, you could just build a small wood fire in it.
A typical gas grill makes way too much heat to barbecue with. The goal is not to churn out hotdogs, burgers, and chicken in a half hour (what gas grills excel at), the goal is to have the meat sitting in the low smoky heat for 8-15 hours.
If you insist on using gas for heat, you probably want to use the smallest burner on the lowest setting, and place the meat away from it (like burning on the right side, with meat on the left… with a sensible cross venting arrangement). Absolutely fine to use a foil pillow of wood chips (punch a few holes) to make smoke. Plan on replacing your smoke pillow hourly, so make a handful of them.