G stands for "Group" - they really need to think of something more original.
G7 (formerly G6)
The G7 ("Group of 7" Yes, really.) are the finance ministers of seven industrialized nations:
France, Germany, Italy, Japan, United Kingdom, United States, and now Canada
The G8 (you can just guess what that stand for) also includes Russia and particularly focuses on the Northern Hemisphere and is a more general "heads of state" and has several subgroupings.
With the G20 we're back to finance ministers and government banker types of these governments:
- Saudi Arabia
- South Africa
- South Korea
- United Kingdom
- United States
- European Union