Ardour (DAW) is both gratis and libre (though supported ready-to-run builds cost at least US$1). Other gratis and libre DAWs include QTractor and LMMS and likely soon ZRythm.

Bespoke, an innovative, idiosyncractic modular music making environment, is also gratis and libre.

The LV2 plugin API has on the order of a thousand gratis and libre plugins that do almost anything you can imagine, and a bunch of stuff you probably cannot, and there are also many VST and AU plugins that are gratis (but frequently not libre).