I will log this issue with the respective developers of the plugins as well but thought I’d mention it here in case anyone can throw any light on it - or in case it’s something Deskew can address.
I have several plugins from SK Note and they will only load as VSTs - they fail to initialise every time as AUs in Gig Performer (but they are fine in Logic).
I also have the wonderful chorus from Fred Anton Corvest - FAC-Chorus - and this is only available as an AU (through the Mac App Store). It also won’t initialise in Gig Performer (although it works fine in Logic).
UPDATE: I just bought the bundle of all four FAC plugins and they also will not load in GP but are fine in Logic.