One gigabit is 0.125 gigabytes — an eighth, as with every bit-to-byte pair. At this scale the conversion belongs to networking hardware, where gigabit is the baseline and the byte figure is what actually reaches the disk.
About the units
A gigabit is 10⁹ bits, a gigabyte 10⁹ bytes. Gigabit Ethernet, standardised in 1998, became the default wired network speed and gives the pair its everyday meaning: 1 Gbit/s is 125 MB/s at the theoretical ceiling. Above it, 10, 25, 40 and 100 gigabit Ethernet are all in production use in data centres, and the conversion factor stays the same throughout. Note that network speeds use decimal prefixes strictly — a gigabit is exactly 10⁹ bits, with no binary variant — which makes this one corner of computing where the SI meaning was never disputed.
The exact factor
The factor is exactly 0.125. A gigabit link transfers 125 MB/s in theory and typically 110 to 118 MB/s in practice once framing is accounted for, which is close enough to a mechanical hard drive's sustained rate that the network stops being the bottleneck. A 10 Gbit link at 1.25 GB/s comfortably outruns any single drive and is limited instead by the storage array behind it.
Where you meet this conversion
Network specification and troubleshooting. Comparing a link's rated speed to observed file transfer performance requires this factor, and forgetting it makes a perfectly healthy connection look eight times too slow. Storage area networks, backup appliances and video production over IP are all specified in gigabits while their workloads are described in gigabytes. Internet exchange traffic and data centre interconnect capacity are quoted in gigabits or terabits per second.