If you look at the available info it appears the best Class D amps measured so far are not GaN. Moral of the story is GaN alone does not necessarily mean best. Hypex and Purifi and a few others are not GaN yet and still seem to measure better and are at or near the top overall.
GaN is clearly a promising new technology but still may be a ways from full realization.
I’ve heard both AGD and Atmasphere GaN amps and both sound very good to me but that is just my subjective assessment. Neither may be most cost effective.