(GigaBytes per SECond, GigaBits per SECond) One billion bytes or one billion bits per second depending on whether the B is upper case or lower case. However, using "B" for byte and "b" for bit is not always followed and often misprinted. See space/time.

Upper Case "B" for Bytes
GB/sec, GBytes/sec, GB/s and GBps means gigabytes per second and is used for the highest-speed data transfer ratings.

Lower Case "b" for Bits
Gb/sec, Gbits/sec, Gb/s or Gbps means gigabits per second and is used for transmission speeds in a network or in internal circuits.