A person dedicated to all forms of gaming.
This includes various genres, niche titles, and unique gaming experiences outside of the mainstream.
Not to be confused with hardcore gamer, which is quickly changing definitions from, "a dedicated gaming enthusiast," to, "a person who plays one or two different games almost obsessively, while decrying all others."
A garfnag is the opposite of a casual gamer (note that urban dictionary's entry for casual gamer is questionable in the eyes of most garfnag).
Most of the people who frequent the Penny Arcade forums (especially the Games and Technology section) are considered garfnag.