As I said in bug #896256 about media-sound/audacious 4.3-beta1 version bump: Please also restore ability to build the GTK UI, now that Audacious supports GTK3 once again, since the Qt one in classic mode is still missing important features like "jump to track" [2]. AFAIK both Qt and GTK can be enabled in a single Audacious meson build so the build don't have to be run twice to compile both. [1]: https://audacious-media-player.org/news/55-audacious-4-3-beta1-released [2]: https://www.redmine.audacious-media-player.org/issues/978
The flussence overlay provides a gtk3 ebuild. This ebuild could be a starting point. https://gpo.zugaina.org/media-sound/audacious
Does in v4.4.1.