What does G standfor and the equivalent?

Answer 1

Not sure what you mean, but the best guess I can make is:

What does the G in Gm, Gg, etc mean - ie as a prefix to a unit.

In this case it is a prefix of Giga- which means multiply the unit by 109 which is the same as 1,000,000,000:

1 Gm = 1 Gigametre = 1,000,000,000 m

1 Gg = 1 Gigagram = 1,000,000,000 g = 1,000,000 kg = 1,000 Mg = 1,000 tonnes = 1 Kilo-tonne

etc.