In the beginning, before there was an official Project Gemini mailing list or even a Project Gemini capsule, absolutely all of the information about the project was hosted in Gopherspace. In order to make the project's history more accessible, some of that early content has now been migrated in Geminispace, and links into Gophespace are provided for stuff which hasn't.
Before the "Gemini" name had been coined, before it was even clear that an actual real life protocol people actually used was going to be the result, Solderpunk wrote a series of phlog posts musing about Gopher, the web, what was drawing people from one to the other, and what interesting possibilities might exist in between. These posts provide some informative background for Project Gemini, but aren't technically a part of it. The links below are Gopher links to posts in Solderpunk's personal phlog, and you'll need a Gopher client to read them:
2019-03-03 - Pondering what's inbetween gopher and the web
2019-03-31 - Why gopher needs crypto
2019-04-07 - More on gopher and crypto
2019-06-12 - The soul of gopher
2019-06-16 - Protocol pondering intensifies, Pt I
2019-06-16 - Protocol pondering intensifies, Pt II
2019-06-16 - Protocol pondering intensifies, Pt III
Once Project Gemini started in earnest, Solderpunk set up a separate, dedicated side-phlog for discussion and communication. These posts definitely are a part of Project Gemini, so they have been minimally translated to gemtext and rehosted here at the official project capsule. They offer some insight into the very early evolution of the protocol. Note that many of these posts include links to Gopher posts made by other people, so you'll still need a Gopher client to get a complete picture.
2019-06-21 - Featherweight text protocols
2019-06-22 - Gemini in the wild, AV-98
2019-06-24 - Gemini in 100 lines
2019-06-24 - Assorted small notes
2019-07-03 - Quick update on link syntax
2019-07-10 - Link syntax, stagnation
2019-07-21 - Link syntax finalised
2019-07-25 - Inputs and client certificates
2019-07-29 - Two proposed status schemes
2019-08-08 - Status codes semi-finalised
2019-08-09 - Request formats, virtual hosting and proxying
2019-08-13 - Announcing Agena, a Gemini-to-Gopher proxy