After having read the PRL paper [1], I am not really sure why they left out gain (and pattern) measurements for the antennas. In my experience, radiation efficiency is only one part in determining the received signal strength. The impedance match is also crucial in determining the effective gain.

The figures in their paper also seem to demonstrate strong resonant coupling effects, which do not always translate to effective radiation.

[1] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114...