How to get icon overlays with git-extensions
GitExtensions is awesome for the true git experience. TortoiseGit obscures Git, forcing it into a "simple" experience. TortoiseGit has icon overlays though. The two work quite nicely together, so just install them both, use them for their strengths, and ignore them for their weaknesses.
I don't see any icon overlay proposed by gitextensions (see its GitHub repo).
So amongst the other GUI for Git on Windows, one of the most complete would be SmartGit (but not free though), and none offers overlays at the Windows Explorer level.