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generally speaking, if they state any claim as to the "gain" of the antenna without any statement of radiation efficiency, at the very least they're being disingenuous, not totally forthcoming with regard to the facts.

here's an example: say they claim a certain antenna has a gain of 5.82 dBi. but the radiation efficiency is only 70%. treat the stated gain as directivity and assume that since no Reff has been given, treat the "gain" figure as such: directivity X radiation efficiency = gain. 5.82 X .70 = 4.074 dBi. (4 dBi.) only when Reff = 100% (not happening) does directivity directly translate to gain.

here's another example from another well known antenna, the avanti astroplane. advertised at 4.46 dBi. gain with a power multiplier ratio of approximately 2.79X. both of those figures are incorrect if Reff is < 100%. seeing as most of these antennas were incorrectly configured and then mounted on conducting support structures without isolating element #14 (also known as "the mast") which is required to be a half wavelength long at the operating frequency for maximum radiation efficiency, this antenna had a Reff well below 100% and once again, both of these numbers were substantially less than advertised.