I also have high expectations for the use of faster switching GaN output transistors in class D amps since they have the potential to eliminate dead-time, which even further reduces distortion, which in turn, typically results in improved sound quality.
I've been using good quality class D amps in my combo music and HT system for almost 6 years now. In my opinion, they're the best option for attaining hi-end amp performance quality at an affordable price.
In my experiences with good quality class D amps, I've discovered their most unique quality is that they have no sound qualities of their own. They just faithfully amplify the inputted signal without adding or subtracting anything. They're like the audio amp ideal of a "straight wire with gain".
Due to this extreme neutrality of class D amps, I've come to the conclusion that the most important factor determining the overall sound quality level of a system utilizing a good quality class D amp is the sound qualities of the preamp chosen to pair with it. This neutrality also means the overall system sound quality contributions made by source components, and even interconnect and power cables, will be readily apparent and need to be chosen carefully, too.
I've not yet listened to a class D amp utilizing the much faster switching GaN transistors but am looking forward to doing so, hopefully, at an affordable price. Based on my experiences with good quality class D amps, however, I suspect the use of GaN transistors will result in these amps being among the most neutral ever produced. Choose your preamp, source components, interconnect and power cables appropriately.
Tim