`git gui` and `gitk` my Swiss-knife tools for every day git-ting. They are maintained by git core team.
It does indeed have the same functionality as gitg for staging and reverting individual lines. It does have several UI niggles. For example when selecting it does not do so on a line by line basis, but instead character by character. But the operations are line by line, (or hunk). You also can't right click on file names and choose stage, revert, or delete.