Rosegarden and NoteEdit seem to be two common choices for gui work on music with a lilypond export in the end. I saw one specifically for lilypond once, which seemed to need a bit of work yet (and I think that project stopped). The LilyPond main developers haven't intended to ever work on a gui if I understand right, but other gui programs with export may be able to do what you need to get a lilypond score fairly quick.
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Rosegarden and NoteEdit seem to be two common choices for gui work on music with a lilypond export in the end. I saw one specifically for lilypond once, which seemed to need a bit of work yet (and I think that project stopped). The LilyPond main developers haven't intended to ever work on a gui if I understand right, but other gui programs with export may be able to do what you need to get a lilypond score fairly quick.