I ended up buying the GC42S after a lot of hesitation and going back to try it. I’ve had it for about 3 months now, I didn’t want to post anything on it until I saw how the sound unfolded. I kept the original strings for about 6 weeks, and am now using a set of Galli carbonio. I intend to try out different strings. So far I am extremely happy with the guitar. Words are blunt instruments to describe de subtleties of sound. I find the sound extremely pleasant, warm and sweet, with the usual variations that guitars offer from day to day for no apparent reason, plus the variations dependent on the state of one’s nails etc. The sound is remarkably even throughout the guitar, with the right amount of resonance and sustain, with none of those unpleasant surprises of notes that stick out for no reason here and there. Unless some misfortune occurs, I intend for this to be my main guitar, practically my only guitar, for the rest of my life.
One thing worth mentioning. Before buying it I checked the accuracy of every note, fret by fret, measuring the frequency with a PitchLab application on my phone. I can say without hesitation that the intonation of this guitar is the closest to perfection I’ve ever seen. I was astonished by this. Most of the notes were within 2 cents of perfect, and the maximum deviations I saw didn't exceed 5 cents. This is something I had never seen before (granted I had never tested this on a guitar as pricey as this one).
I found an interview with the luthier whose signature appears on the label, Mr Akio Naniki. The interviewer makes the introduction in Thai, and then the interview is conducted in English, though Mr Naniki’s English is rather limited and difficult to understand. He says the wood (for the top) comes from Europe. He says his apprenticeship took place under Mr Kato.https://www.youtube.com/watch?v=JGdXtebuBIo
I don’t know how much exactly Mr Naniki participated in the making of my particular instrument, but I am very very happy with this guitar.