GaN = Gallium Nitride
Gallium Nitride can be used to make a more efficient switching power supply. A lot of the newer, smaller wall warts for laptops use it (Anker makes a few models with GaN: https://us.anker.com/pages/uai2020
GaN has been used in industrial power supplies for a while as well. Here's an application note describing the advantages (size and efficiency) compared to traditional MOSFETS using silicon. https://epc-co.com/epc/Portals/0/epc/documents/product-training/appnote_ganfundamentals.pdf
Peachtree Audio is selling a class D amp using GaN (https://www.peachtreeaudio.com/products/gan400
). They asked someone to write up impressions, and the writer says that most class D amplifiers "sound wretched," but the Peachtree GaN amp is the best of tubes and solid state. The review doesn't address good class D amps like the Cherry.
I haven't seen comparisons between the Cherry amps and a class D amp using GaN. Maybe GaN is incredible, but it's been around a long time and has made its way into mass market wall warts, so its use in more hifi amps would seem to be overdue if the advantages were overwhelming. Are there amps that have been more substantively reviewed than the Peachtree which use GaN? Has anyone heard one in direct comparison with the Cherry amps?