In general, I would talk about antenna efficiency instead of radiation efficiency. Antenna efficiency is a parameter that accounts for losses in your antenna. This is dimensionless parameter that is given by multiplication of three efficiences: reflection efficiency, conduction efficiency and dielectric efficiency (by multiplication of the last two you get radiation efficiency).
So if the losses in your antenna are high, e.g. your antenna is poorly matched, or the power is lost in conductor or dielectric, it doesn't matter high directivity of the antenna, you will get low gain.
Directivity is also a dimensionless quantity.
Hope I helped