GB is an abbreviation for gigabytes, this is a measure of data generally used to specific storage space on a platform or as a level of traffic for bandwidth, and is equal to roughly one billion bytes, or 1,073,741,824 bytes to be exact. Another measure for a gigabyte is that it is traditionally equal to 1,024 megabytes (MB). Dacast does a simpler metric where a GB is equal to 1000 MB, and similarly, a TB is equal to 1000 GB.
In terms of how this relates to Dacast, there are two ways. The first is from bandwidth, which refers to the amount of traffic caused by streaming (higher audience and higher bitrate translate into more bandwidth being consumed). The second is from the amount of storage given to accounts, which is measured in terms of GB.