In data communication or computer industry, Gb/s, GB/s or GT/s is used to describe the data transferring rate or bandwidth between endpoints. Gb/s is gigabits per second, GB/s is gigabytes per second, while GT/s is gigatransfers per second. The conversion between Gb/s and GB/s is simple: dividing Gb/s by 8 will get GB/s.
To convert GT/s to Gb/s, the encoding of the data has to be looked at. For example, PCIe 2.0, it mentions the interconnect bit rate is 5 GT/s. Considering that encoding scheme in PCIe is 8b/1b, the effective bit rate is 5 Gb/s * (8/10) = 4 Gb/s. Basically, GT/s is raw data rate, while Gb/s is effective data rate.