6 Gbps, 600 MBps (the capital B is supposed to indicate 'Bytes' versus 'bits') the encoding is 8b/10b which is 10 bauds per 8 bit byte.
PCIe 2.0 has 2.5Gbps "lanes" PCIe 3.0 has 5Gbps "lanes" they can be ganged together for additional bandwidth. (x1, x2, x4, x8, x16) it is also 8b/10b so you divide by 10 to get Bytes per second (250MBps/500MBps).
Both SATA and PCIe have a 'transaction limit' which is a function of the controller, which limits the total number of operations per second (IOPs). The product of the IOPs and the size of the transaction can never exceed the bandwidth of the channel. But it often is under. For example a typical SATA disk control (prior to the popularity of SSDs) would do about 25,000 IOPs, and if you had 512 byte (.5K) block reads and writes, you could read and write 25,000 * .5 or 12,500K or 12 MBps (which was much lower than the theoretical bandwidth of 200MBps on 2Gbps SATA II channels. Optimizing channel utilization requires that you figure out how many IOPs your OS/Controller can initiate and then sizing the payload to consume the max bandwidth. Large payloads and you'll push IOPs down, smaller payloads and you won't use all the bandwidth.
One of the nicer aspects of ATM was that it was designed and specified for full channel utilization with 64 byte packets which made it possible to reason about the performance and latency of an arbitrary number of streams of data moving through it.