The National Tribune, Thursday, January 9th, 1890 [Volume IX, NO. 23, WHOLE NO. 439], in a front page serialized story segment entitled, Story of a Cannoneer, Reminiscences of a Detached Volunteer in a Regular Battery..., (Author, Augustus C. Buell?), (story, copyright 1889), fifth column, on the bottom:
Not having been with the battery in its
earlier battles, I could not judge what was the
spirit of its men at Manassas, Antietam or
Fredericksburg; but so far as my limited ob-
servation enabled me to judge, I should say
that the veterans were “away up in G,” as
musicians say, on that occasion. there is
no doubt but that if the enemy had brought up
another battery it would have shared the fate
of the one which stood there silent and dis-
mantled in front of our still smoking muzzles.
I looked, but failed to find the phrase in the book, The Cannoneer: Recollections of Service in the Army of the Potomac, by Augustus C. Buell, copyright 1890, by The National Tribune. It should have appeared somewhere on pp. 214 to 216. I was unable to locate an 1889 edition.
The difficulty of playing in different keys depends upon the construction of the instrument and the skill level of the musician. Some instruments have a limited range and are built to be played broadly in only one key. The most common harmonica is a good example but there are also other types of harmonica that are not so limited. The valveless bugle is another example. At the other extreme is the trombone with its slide, limited only by the instrument’s range and technical skill of the player.
From a purely auditory viewpoint, there is no difference between keys. Computers can transpose music up and down into any key (and even between keys). Also, from a compositional viewpoint that does not take limitations of instrumentation and technical skill of players into account, there is no difference between keys.
Of course, difficulty may be isolated from “difficult to play in key.” For example, John Cage’s Freeman Etudes for violin were written intentionally to be just at or beyond the edge of possibility for a virtuoso player to perform. Ordinary master players are not easily able to play the pieces. Cage said in a 1983 interview: “These are intentionally as difficult as I can make them, because I think we’re now surrounded by very serious problems in the society, and we tend to think that the situation is hopeless and that it’s just impossible to do something that will make everything turn out properly. So I think that this music, which is almost impossible, gives an instance of the practicality of the impossible.”
I am unaware of the key of G being notoriously difficult to play in.
Ignoring solfege, a WAG: the origin of the phrase may be related to the perception of the progression of the “A B C’s” of the alphabet, in the following way: the musical notes of the common modern Western 12tet (twelve-tone equal temperament) scale are commonly named A B C D E F G. G would seem to be as far away from A as one could get in a listing, non-musical sense that conflates the linear progression of A through Z to A through G. But if so, wouldn’t it make sense for it to be “away down in G?” I don’t think so; after all, it is slang.
I’m sure I’m missing something, but I’m interested enough to keep an eye open for more on the origin of this phrase.