Definitions of Gb:

noun: a unit of magnetomotive force equal to 0.7958 ampere-turns
noun: a highly toxic chemical nerve agent that inhibits the activity of cholinesterase
noun: a unit of information equal to one billion (1,073,741,824) bytes or 1024 megabytes
noun: a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland