G is the SI prefix for "Giga", meaning 10^9. So, 1Gm is the SI representation of "1 giga meter = 1 billion meters", for example. It's most commonly used in computing, e.g. in 1GHz = 1 billion Hertz. You may have also seen it in power output numbers, such as saying that some power plant produces 1GWh annually (1 giga watt-hours = 1 billion watt-hours).
Of course, in computing G is also sometimes used to mean 2^30. The ISO has allocated the prefixes Ki/Mi/Gi (Kibi, Mebi, Gibi) for that use, so technically my machine has 16 GiB or RAM, which is slightly more than 16GB of RAM would be.
Of course, in computing G is also sometimes used to mean 2^30. The ISO has allocated the prefixes Ki/Mi/Gi (Kibi, Mebi, Gibi) for that use, so technically my machine has 16 GiB or RAM, which is slightly more than 16GB of RAM would be.