Myself, I miss how "G" was refined into "C" after following Carolyn Kunin's initial arguments: "The index title page tells us that the three main characters of the book are G, K, & S. After staring at those three letters for quite some time I began to see the G as a modified C and then the solution came to me: C = K = S."
Dear Jansy,
All I can tell you is that when I looked at the three letters I suspected there was some link between them. I think the answer to your question is that VN hid the "C" inside the "G" while making the joke that your yourself seem to have guessed, that G = C minus. This may also explain the "Karly Garly" song as Victor Fet seems to have in mind.
Carolyn
p.s. This reminds me of the joke that when G_d had finished creating the world he saw that it was "very good" - - that is, he gave it a B+.
In another vein I once heard of a white cat who was given the name C Major. She produced a kitten with a few black spots who was named C Minor.