User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-4132454-20120219193649/@comment-1011548-20120219194755

G-Rank, or G Lvl in MHFU, is just the 3rd level of difficulty. Basically, Elder Quest / Early Guild Hall is easiest, High Rank is medium, G Rank is hard. The monsters have a great deal of health, might move slightly faster, and may even have new attacks. Portable 3rd didn't have G Rank, unfortunately.