I have yet to hear any GaN Fet amp design, so can’t comment on the sound of those. My comments have more to do with my class D journey--how I proceeded amidst the pro- and con- hype.
First: I know my own sonic tastes and preferences very well. I prefer amplification that is slightly warm, definitely spacious & resonant, has a midrange that is expressive and full, and conveys the natural weight of lower frequency instruments. In short: sound that is "musical" (sounding a lot like the real thing IRL), not sound that is forensic and unnaturally detailed.
Second: I addressed my own ignorance by reading vast amounts of reviews and user comments for dozens of class D amps, new and old. At first I wasn’t much concerned with cost or availablity, but more with sound. This took months, mainly because relatively few reviews really characterized the sound of amps in ways I recognize; and even fewer user comments drilled into that side of the amp in question. Instead, there was a lot whiz/bang stuff about S/N ratio, damping fact, amperage into low impedance loads, blah-blah. Those things are obviously important, but lacking characterization of the amp’s sound, they don’t mean as much to me.
Third: I finally identified three or four class D designs that were (more frequently than others) lauded as sounding like music. The first was an ICEpower design, the Wyred 4 Sound ST-500 class D amp. I found one used and liked the sound a lot right away. I felt extremely lucky that all my homework had paid off. Next I picked up a used pair of Bel Canto 600M NCore monoblocks. Again, I liked the sound right away. Another huge piece of luck.
So I was able to find one example each of two legacy class D technologies (ICEPower and NCore) that sound really to me and were affordable. I suspect there’s at least one GaN FET design out there would fit the bill, too. My research is ongoing, but I have yet to find any affordable names to get interested in, plus the two I have (ST-500 is backup; 600Ms are in the system) are so good, I really have no need to upgrade.
PS: what started me on all this was the limitations of space I face in this home offic.e There is simply no room for an A/B class stereo amp here, much less a paif of A/B monoblocks. Class D definitely works here, with little if any heat output and far more compact case footprints.