You can use wet wood chips and foil or a special smoker box to smoke fish on your gas grill. This method will impart similar smoky tones to fish as charcoal grills and smokers achieve.
Advance Preparation of Fish and Wood Chips
Mix 1 gallon of water, kosher salt and sugar in a large plastic bowl to create a brine solution. Place the fish in the brine solution. Cover the container with plastic wrap, place it in the refrigerator and allow the mixture to soak overnight.
Add the second gallon of water to the large mixing bowl or bucket and soak the wood chips for at least an hour before beginning the smoking process. Wet wood burns longer and produces more flavor-enhancing smoke than dry wood.
Place a handful or two of the saturated wood chips inside the smoker box or in the center of the aluminum foil sheet. If you are using foil, fold it over the wood chips several times to create a foil pouch. Use a fork to create about a dozen vent holes in the pouch.
The Smoking Process
Light one gas burner on the extreme left or right of the grill. Leave all other burners off.
Adjust the lit burner to medium-high and place the smoker box or foil pouch underneath the cooking grate on or near the lit burner. Close the grill lid and allow the box or pouch to heat until the chips start to smoke.
Reduce the heat to medium-low or low to achieve a grilling chamber temperature of 140 to 160 degrees F.
Place the brined fish on the cooking grate above the unlit burners and close the lid tightly.
Check the smoker box or foil pouch every 30 minutes to ensure smoke continues to billow. Replace old chips with fresh wet chips if smoking ceases.
Continue to smoke the fish for about two hours between 140 and 160 degrees, depending on the size of the fillet. Keep in mind that smoking your fish for too long will result in dried-out, tough fish.
Don't skip the brining steps. Brining your fish helps lock in moisture and keeps it from drying out during the long, slow smoking process. Milder hardwoods, including alder and cherry, complement fish better than richer woods, such as hickory and mesquite.
Always use fire-resistant cooking mitts when opening or closing hot grills and handling hot foil pouches or smoker boxes. Handle fish with a long-handled grilling-style spatula to avoid burns.