Grit was the name of a _Ruby_ implementation of git way back when: https://github.com/mojombo/grit/. I believe it's actually what GitHub was built on then.
I created and named the Grit library that used to power GitHub. Scott Chacon (fellow GitHub cofounder, now CEO of GitButler) specifically asked my permission to re-use Grit as the name of this project, which I gladly granted. R is for Ruby. R is for Rust! Grit is dead. Long live Grit!
I started the project as Gust, but felt like Grit was such a better name. I asked Tom if I could boot the name back up again because I always liked it and he said it was fine.
Also, I worked on the Ruby Grit pretty extensively during the early days of GitHub, so hopefully I earned the right to carry on the mantle. :)