Gig Performer exposes a single Audio Out block per instance. You can’t add a second independent Audio Out plugin within the same instance, regardless of whether you’re on Windows or macOS.
GP Relayer doesn’t change that architecture; it’s designed to transport audio and MIDI between Gig Performer instances and/or DAWs, not to create additional output devices within a single instance.
Are you using OBS? If so, you’d want to take a look at this article: [blog] How to set up live streaming using OBS, Gig Performer and JACK router