G meaning in Space Science ?

Giga ( or ) is a unit prefix in the metric system denoting a factor of a short-scale billion or long-scale milliard (109 or 1000000000). It has the symbol G.

Giga is derived from the Greek word γίγας (gígas), meaning "giant." The Oxford English Dictionary reports the earliest written use of giga in this sense to be in the Reports of the IUPAC 14th Conference in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 109×."

When referring to information units in computing, such as gigabyte, giga may sometimes mean 1073741824 (230); this causes ambiguity. Standards organizations discourage this and use giga- to refer to 109 in this context too. Gigabit is only rarely used with the binary interpretation of the prefix. The binary prefix gibi has been adopted for 230, while reserving giga exclusively for the metric definition.