I had some issues with crashes when I used Omnisphere in Gig Performer.
Together with support of Spectrasonics I found out that using mixed versions on a gig is a not clever idea.
On Mac one should only use AU Version.
Maybe VST is working too, but never mix AU and VST!
And another issue I faced (no crash) when using mixed versions.
Sometimes it could happen, that the GUI of the AU version is not shown.
Instead Gig Performer tries to display the parameters somehow.
This happens when the the GUI of the VST version is opened and then one tries to open the GUI of the AU version.