Metalguy, MIG (Metal Inert Gas) was changed to GMAW because some of the gasses being used are NOT inert, such as carbon dioxide & oxygen. Hope this helps.
W. Baker
Just progress many of the old names were simplified or traenames - when first used, GTAW aka TIG was trademarked by Linde Corp as Heliarc welding as the inert gas used was helium. I belive now argon is the most widely used (by percentage) gas and other companies offer the process so the name had to be changed anyway.
Accepting all the previous replies, in U.K. this process is also known as Tungsten Active Gas as the more modern gas mixtures can actually add to the weld constituent.
rarely is weld porosity (gas inclusions an reacted components) desireable in modern welding as a joining process. it may be used in various flame spraying methods but not in joining.
gases that combines with the weldment are generally used because of cost rather than the quality of the weld.