The G tuning brings out a special/different feel that you don't hear with a traditional "C" tuned box. Not to say that the traditional "C and "D" tunings aren't great. The G just brings something different to the table.
Follow this link and listen:
https://www.youtube.com/watch?v=g5XL6yURgGc
As you say, G is one of the lesser used keys. But, if you check it out, it has its place.
In zydeco, it is used much more these days than in the past. Part of what makes it work is that when you play in the "pull" or 2nd position, you are in the key of D. Most males are bass/baritones, and the key of D lends itself to singing in a more relaxed or soulful way than the higher keys.
I bought a Gabbi in G thinking it would be good for singing but in the majority of cases the C box wins and I'm no virtuoso singer. The only real joy I've had is being able to sing Bonsoir Moreau, otherwise its a dreary sound and lacks the liveliness of a C, not bad for low down dirty blues though to be fair.
Thinking I'll get C hohner reeds put in as I like the box and I've sold my 114. If SOTM gets resurrected I'll post a tune on G Gabbi as its' my main player at the moment.