Gas grills are less messy in general; as there is no bi-product, because there is no combustion process, they are easier to clean so all you need to worry about is mess from the food. This also means that they can be used again with a minimum of effort on your part.
You also have more control over your cooking because you can turn down gas grills. This can be a handy advantage over charcoal burners, where you pretty much have to let things take their course. Gas grills also heat up almost instantly, compared to charcoal burners which can take an irritatingly long time to heat up.