GSequencer

GSequencer is a music sequencer built with Gtk+. It covers the ordinary tasks of pattern-based composition: entering note events, grouping them into patterns, assigning those patterns to tracks fed by instruments or effects, and playing the arrangement back on a timeline that can be edited as playback continues.

The publisher's summary describes it as an advanced sequencer. What that covers in any given release — the audio engine, the plugin and project file formats accepted, the depth of the mixing and automation facilities — is set out in the project's own documentation, and is worth checking against the specific version on offer rather than assumed to match another host.

Joël Krähemann releases it under GPL-3.0-or-later. Linux is the only platform listed. Since a sequencer depends closely on the system audio stack and its latency configuration, this makes it a fit for Linux audio setups rather than for mixed-platform studios.