Hi Joseph (hope that’s the right name, if not, please correct me ),
well, GNU Radio is software defined radio; the data it processes are
simply numbers, so its unit is 1.
If your signal comes from an ADC, then it might be proportional to
Volts at the receiver; however, you usually do a lot of processing after
receiving, so using Volts as a unit is kind of wrong, because none of
the components consider these numbers to represent a voltage. If you
happen to square your signal, the display would be proportional to
power, instead, so there’s no way to tell you "the scope sink displays
physical entity "; it all depends on what you let it display.
I hope that helped a little,