I have been following the news of this for a while now, and at first it seemed like G was simply the US version for Japanese release, as they have had the game for a lot longer than we have. The updated G previews mentioned Dual Wielding and being able to see equipped weapons in Towns, two features which we already have in our release. Upon further investigation however, there were a few things which appeared to be new. I can't say with certainty that G will be a rehash of the North American release.
note: as of this post, the images Crush linked to appear to have been taken down, here is a link to
The Japanese Monster Hunter G site.