It reflects the data rate. Since DDR memory transfers data on both the up and down part of the clock signal, DDR RAM on a 3000Mhz clock signal is said to make 6000 Megatransfers per second, in normal usage. 48 GT/s would imply a 24Ghz clock if it were normal DDR, which seems absurd.

Edit: It seems GDDR6 is in reality "quad data rate" memory, and GDDR7 packs even more bits in per clock using PAM3 signaling, so if I'm reading this right maybe they're saying the chips can run at up to 8Ghz base clock? 8ghz * 6 bits per cycle * 32 bit bus / 8 bits per byte = 192GB/s.

Edit again: It seems I undercounted the number of bits/pin per cycle of base clock for GDDR7 and it's more like 12 (so 4ghz max base clock) or even 24 (so 2ghz), which seems a lot more reasonable.

Gigatransfers per second (GT/s) measures the rate of data transfers rather than the data rate itself. Each "transfer" represents one unit of data moved across a data bus per clock cycle, which can be thought of as one signal transition on the bus.