It's common to find wasps using your gas barbecue grill as a home, especially if you haven't used the grill in a long period of time. The barbecue gives wasps a safe area in which they can live and build nests. This problem, of course, isn't conducive to cooking on the grill. As such, you can get rid of these wasps in a number of ways.
Light the gas grill if you can access it safely without disturbing the wasps. This method is the most effective to get rid of wasps in the grill itself; the fire will kill some, make others flee and destroy any nests the wasps have built.
Spray the entire gas grill with a pressure washer or a hose with a power nozzle to destroy the wasp nest and discourage the wasps from congregating in your grill. Though water is not fatal to wasps, it will cause them to fly away and find somewhere else to gather. Water is also beneficial because it will not damage your grill any worse than a rainstorm would.
Treat the areas of the gas grill contaminated by wasps with a sprayable wasp repellent. This treatment kills some wasps instantly, poisons others and causes the rest to leave the area. Once the wasps are gone, remove any nest left behind and dispose of it by crushing it or burning it. The downside to this treatment is the toxicity of the wasp repellent; after using it, you'll have to thoroughly wash your grill.