GPC is an economy (or generic) brand of cigarettes produced by the Brown and Williamson Tobacco Company. First created in 1985 as a private label plain packaged (black and white) brand -- they have grown to include 21 different styles. In 1989 color was added to the packaging and in 1991 the flip-top box was introduced. They have a current market share of 4.7%, making them the seventh most popular brand in the U.S.
I don't believe an "official" meaning behind the letters G-P-C has ever been released by the company, but many people assume it means "General Purpose Cigarettes". TBBK says he always thought it meant "Generic Product Council". This latter explanation makes sense also because the original packaging at least used to say "GPC approved". This latter wording would make more sense in this context. Neither phrase appears in a search of the internet which makes me think that it's likely that neither phrase is the "correct" one. Many other people have created their own humorous guesses including "Good, & Pretty Cheap", "Generically Packaged Camels", and "Ghetto Peoples' Cigarettes".
Update 1-Oct-02: Mystery solved. I received a response from a Ms. Brenda Manring of Brown and Williamson. She says, "GPC does not stand for anything. It's just the brand name."
GPC's styles include: Full Flavor, Light, Ultra-Light, Menthol Full Flavor, Menthol Light, Menthol Ultra-Light, Non-Filter. Most of the styles are available in either King Size or 100's and in soft packs or boxes.