Most information is in the README and DESIGN documents, so be sure to read those. See also the web page on hg-git.github.com.
Currently, this is mostly working – gclone works pretty well and gpush basically works but doesn’t really tidy up well, I need to clean that up. This project is actually pushed via Hg-Git, it hasn’t been touched by Git yet on my system, so anything here got here via Hg-Git.
The plugin is not stabilized yet, so I may (will) change the conversion formats, meaning objects you push now will not convert back to Hg the same and aren’t going to be the same as they will when you do the same thing a week from now. In a few weeks this should all be resolved to a stable format.