If identical high-quality steaks are broiled at the same, very high temperature (700F or more), one over charcoal and the other over natural gas or propane, then they will taste the same to most people.  In short, there is little difference in the taste of food which is broiled with charcoal or gas, and many people (especially gas grill sellers) use this to argue that gas grills are just as good as charcoal grills for grilling and even barbecuing.

However, this example does not prove that grilling with charcoal imparts no flavor.  To the contrary:  it imparts a unique flavor.  But the simple reason that most people cannot tell the difference in the flavor of the two steaks is that high-quality steaks are cooked for a very short period of time -- too short for the flavors of wood charcoal to be noticeable in the taste of the steak.

Notice all the qualifications, like "very high temperature", "high quality" steaks, and "high-quality" charcoal. These qualifications result in broiling the meat (i.e. cooking almost entirely with radiant heat) in a very short time, so that the wood smoke has little or no effect on the meat itself.

Therefore, within the narrow range of grilling or broiling high-quality meat, both gas and charcoal broil steaks which have almost the same taste.

But this is a very narrow exception. Any conclusion or implication that there is no difference between gas and charcoal (implicit in the example) in imparting flavor to meat in most grilling and all barbecuing is entirely wrong.

When grilling meat for longer periods of time than the above steak example, the longer the meat stays on the grill, the more smoke flavor will be imparted to the meat and hence the taste differences between gas grilling and wood or charcoal grilling will become more distinct.  For example, if one were to grill a whole chicken on a grill, this is a process that will require an hour or more.  the taste of the finished chicken will be quite different if one uses wood vs. using gas.  The gas grilled chicken will lack that delicious "wood smoke flavor".

Barbecuing

Barbecuing is a method of cooking with the dry heat of a wood fire at a temperature of between 180F and 250F.  Barbecuing is a method of slow cooking, and meat may require 12 hours or more to cook.  When food is cooked in the dry heat of a wood fire for so many hours, complex chemical reactions occur which impart unique flavors to the food.

Gas cooking alone cannot duplicate these flavors.  When gas burns, it does not produce the complex smoke of burning wood.  Without the hot smoke of wood, the meat tastes very different from barbecued food.

Some barbecue restaurants use gas ovens to barbecue meat.  In these commercial restaurant smokers, the heat is produced by burning natural gas or propane and chips, chunks or logs of hardwood are added to the gas fire to provide the wood-smoked flavor.  To the purist, this method of cooking barbecue is a sacrilege, as they feel that to produce true barbecue, the heat must come from only the burning of wood or charcoal. To most people, the taste difference is not discernable between the two cooking methods. Barbecue restaurants cook with gas because the cost of doing so is much less than fueling the smoker entirely with wood or charcoal.  In the end, it comes down to what the customer likes.  There are many successful barbecue restaurants that use smokers fueled with gas with hardwood added to provide the wood smoke flavor.

There are some home smokers that use gas to produce the heat and wood chips to produce the wood smoke flavor.  It is the position of the California Barbecue Association that these smokers should be avoided by people wishing to make "Authentic American Barbecue".  We feel that if you are going to go the the trouble of smoking a piece of meat, you should do it with a wood or charcoal fire.

In competition barbecue, the rules make the use of gas illegal.

Conclusion

A gas grill may be an adequate substitute for a charcoal grill for broiling hamburgers and steaks.  A gas grill also serves as an adequate outdoor oven when used for roasting meat.  Thus, cooking with gas is okay for broiling and roasting.

However, a gas grill cannot be used for barbecuing or hot smoking, since both require the "low 'n slow" heat of a wood fire.  So cooking with gas is a big no-no for barbecuing and smoking.