A gigabyte is 109 or 1,000,000,000 bytes.
One gigabyte (abbreviated “GB”) is equal to 1,000 megabytes and precedes the terabyte unit of measurement. While a gigabyte is technically 1,000,000,000 bytes, in some cases, gigabytes are used synonymously with gibibytes, which contain 1,073,741,824 bytes (1024 x 1,024 x 1,024 bytes).
Gigabytes, sometimes abbreviated “gigs,” are often used to measure storage capacity. For example, a standard DVD can hold 4.7 gigabytes of data. An SSD might hold 256 GB, and a hard drive may have a storage capacity of 750 GB. Storage devices that hold 1,000 GB of data or more are typically measured in terabytes.
RAM is also usually measured in gigabytes. For example, a desktop computer may come with 16 GB of system RAM and 2 GB of video RAM. A tablet may only require 1 GB of system RAM since portable apps typically do not require as much memory as desktop applications.
NOTE: You can view a list of all the units of measurement used for measuring data storage.
Source: http://www.techterms.com/definition/gigabyte