I have no idea how many there are at this point but I know of two developers of excellent plugins who either are or will soon be shipping AUv3s (and with no VST alternative).
Fred Anton Corvest (FAC) - very widely respected on iOS as a maker of exceptionally good plugins - is already shipping some through the Mac App Store. Chorus and Alteza are standouts.
Blue Mangoo - also from the iOS world and maker of the iFretless series of apps among many others - is about to ship Attack Softener, an extremely fast and accurate envelope/transient shaper that works really well on guitar (but is really handy to throw at any sound).
I have no idea what effort is involved in making AUv3s work in GP but you can see them in Plugin Manager - they just fail to validate.
I’m just wondering if there is any appetite to make this happen - and if so, roughly when that might be?