GT/s = gigatranfers per second.
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Also the speed is per lane, eg an x8 slot / port / device is called that because it has 8 lanes, which all transfer in parallel.
Though the exact details of the overhead don't matter very much. They add 6% extra bits, good enough.
The part I want to call out as complicated/confusing is that a PCIe 7.0 lane puts out a voltage 64 billion times per second, but because each voltage is based on two bits that counts as 128 billion "transfers".