I have not listened to the Technics SU-G30 yet. By reading the product page, it is evident that this is a fairly complex device, and that GaN transistors are a small component of the overall design. It is outwawrdly not possible to draw definite conclusions as of the relative contribution of GaN to the performance of Technics SU-G30
Worth pointing out that GaN transistors are not particularly new. Prototypes have been demonstrated as early as 1993... That is some 25 years ago. GaN-based transistors have been commercially available in one incarnation or another since 2006. See:
Nor GaN are the exclusive domain of Niche companies: major semiconductors fabs, such as Panasonic Industrial Devices, offer Gan transistors and GaN-based assemblies:
Relatively novel is only the application of Gan to high end audio amplification. If more amplifier designers adopt GaN, The next couple years will tell if transistors based on some forms of GaN inherently correlate to class D modules and circuits that contribute to enhance the audible performance of power amplifiers beyond the current state of the art. Or if the advantage afforded by the application of this semiconductor technology is purely a theoretical one. The danger of overoptimistic disconnects was nicely expressed by Mr. Berra:
"In theory there is no difference between theory and practice. In practice there is."