Yes they can but they only come in N channel, no P, so you can't do a usual complimentary push pull.
Same reason but more so the P channel in normal Mosfet amp is not as good good as the N but with GaN it would be even more so.
From memory Solution make and class a/b amp using just n channel Mosfets top and bottom, but there are other disadvantages doing this.
Cheers George