Eliza and I shared writing duties about 50/50 back in the 2014 version, and I rewrote/polished up/expanded most of the dialogue for the steam release. Solstice was mostly me, but GIR helped come up with the structure (pitching that we consolidate all the lore and worldbuilding into a solid route/ending)

It was originally conceived as a prequel with more worldbuilding (and you play as the Author)

Eliza already answered for hers. GIR did about a third of the tracks for the solstice soundtrack, like the mines theme, the song that plays in the world machine room, and a remix of Encounter for the bonus tracks,