Gig Performer is an audio plugin host that supports AudioUnit (AU) plugins, VST3 and VST plugins. AU plugins are Mac-only:

WhispAir, AudioUnit plugin, AU plugin in Gig Performer

Generally, if you install or update an Audio Unit plugin, you have to log out or possibly even reboot your computer before the updated AU becomes visible.

If you don’t do this, the old AU will still be registered and so Gig Performer will not find the new one.

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Related topics:
Tips to troubleshoot your computer-based setup
Gig Performer cross-platform usage