It is bits. It's full duplex, each side has dedicated receive and transmit lanes. And for example PCIe x16 is 16 lanes. Each transfer is an encoded bit. PCIe 3.0 and 4.0 require 130 bits to encode 128 bits (PCIe 2.0 required 10 for 8). So 16 GT/s would be ~1.97 GB/s (in Bytes). The primary thing this standard is doing is stepping up the communication speed using better chips (which is why it's so easily backwards compatible).