Giga is a prefix that is used as a multiplier. The prefix giga is written as a uppercase "G" in front of the measure.
In the "analog" world Giga represents the numeric value of 1,000,000,000 (a thousand million or 109); such as a frequency of 5 GHz, which is 5,000,000,000 Hz.
When used in the digital world, Giga represents the numeric value 1,073,741,824 (1024×1024×1024 or 230; e.g. a bit rate of 8 Gbit/s, which is a rate of 8,589,934,592 bits per second).