Thinking about this some more, the question is wrong. As pointed out, there are several appropriate choices of gauge for grouse. The real question is, what kind of gun is best for grouse?
Shooting grouse, especially ruffed grouse, is a game of speed. The birds themselves and the terrain they live in dictate that shooting techniques like sustained lead or pull ahead are seldom successful. The birds are too fast and disappear into the bush too quickly. Instinctive shooting is often all that can be utilized, but even other techniques will have to put a premium on quickness.
In order for a shotgun to be a successful instinctive shooter, it must possess several qualities. First and foremost is gun fit, instinctive shooting with an ill fitting gun is frustrating. After that, things get more interesting.
There is a theory, which I hold to be true, that states it is easier to point a shotgun when the barrels are set low in the hand. The higher the barrels are above your hand, the less instinctive they are to point. SxS's and some U/O's therefore have a natural advantage over most pumps and autos which place the barrels well above the hand.
Another important quality is handling - how easy is it to get the gun pointing where you want? Conventional wisdom on this topic suggests that half or more of the total mass of the gun be located between the shooter's hands. The more centrally located the majority of the mass is, the easier it becomes to handle the gun - up to the point where things get whippy. A break action gun, or a falling block action will naturally concentrate more of the mass between the hands than a pump or auto with their longer frames and more spread out operating bits.
Another factor is not the distribution of mass, but the total mass. I don't care how strong you are, you can move a six lb. gun from port arms to shoulder faster than you can move an 8 lb. gun. The lighter the better for grouse, up to the point where your gun handling skills cannot adequately control the gun. Very lightweight guns take discipline to shoot.
The ideal grouse gun, therefore, has reasonably low mass, has that mass compactly centered between the hands, keeps the barrels as low in the hands as possible and fits the shooter well. The gauge does not matter, except that the load be enough for the game and the pattern be dense and even enough at the distances required.
Factors such as barrel length, number of triggers, pistol vs. straight grip, rib type etc. also come in to play, but everyone has their own theories on those things. The mass / balance / handling equations are all pure physics and physiology.
These things have been studied for a long, long time and are continually refined. They result in what is commonly called a game gun - a gun built specifically for the uplands. Really great ones are rare and expensive, but merely good ones can be had by anyone, so long as you know what you are looking for.
Just my $0.02
Sharptail