GaN FETs are indeed (one of) the frontier tech in switching power supplies. They're pain in the rear to design properly, and if you look at them the wrong way they'll instantly blows up.
But when done properly they offer very low on resistance and gate charge, and most of the packages also minimize parasitic inductance. That enable them to be operated at very high frequency, greatly reducing component size without sacrificing efficiency.
By the way, does anyone make 500V GaN FET?