Comment by insin

In the example of how they were removing specific lines from the previous commit, git gui had a way to do that instead of copying the diff lines and manually editing them back into the code:

git gui → Amend Last Commit → (select lines from a file in the Staged Changes area) → Unstage Lines From Commit → Commit

I still think it's the perfect does-just-enough GUI when the main thing you want is to visually craft commits.