[namibj]

It's the normal unit/measurement of spectral efficiency. You have X Hz bandwidth, and you multiply it by the spectral efficiency to get the data rate.

The fact that the time-unit cancels out just shows that spectral efficiency is independent of any sense of time one might have.

Measuring "spacing" in GHz makes sense if you consider the way heterodyne mixing shifts the signal around, without affecting the spacing of sub-carriers.