Scorewriter

Basically a GUI music editor where you can specifically see and export classical music notation instead of tablature-style notation.

Best open source one found so far as of 2020: MuseScore.

This software feels amazing. You can really start composing very quickly, lots of features, good keyboard shortcuts.

GPL, and there's a backing company that makes money with an online and mobile version of it.

Can export to:

Ubuntu 20.04: sound preview worked, first hat that trailing Contra-like sound artifacts (like spd-say), but then it went away?

Feels like a lot of effort was put into usability, including keyboard shortcuts by default, seems like a powerful and easy to start using software to compose music!

Glissando: yes: musescore.org/en/handbook/arpeggios-and-glissandi

Just use MuseScore instead.

Weight: light.

Can import from: MIDI.

Can export to:

Ubuntu 20.04: tuxguitar-jsa was needed, otherwise no sound: askubuntu.com/questions/457321/tuxguitar-no-sound-in-14-04

Has OK step sequencer non-realtime up/down/left/right guitar based composition interface.

Has chord insertion.

Has bend editor.

Could be more amazing, but it is OK.

A bit limited by being very "guitar oriented". Shows you guitar strings, and you enter offset to each string. So to enter two adjacent notes you need to use two seprate strings and thing about the offsets. If only it had a more piano based interface.

Drum notation is also atrocious, you have to go to the top chord, and use high numbers starting at 36.