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.
Is non-ISO unit "T" / "transfer" a marketing term or really specialised jargon? "transfer" just doesn't click in my mind, at best "a transfer" (countable) is about moving a sizeable aggregate chunk that has some semantic meaning, not a single fundamental quantum of information.
Unrelated: "gigatesla per second" is such a mind-boggling unit.