Point the antennas directly at eachother.
Assume you know the following:
Transmit power (Pt)
Receive Power (Pr)
Distance (r)
Wavelength(l)
and assuming the gain on each is the same, you get
G = ((Pr/Pt)^0.5)*4*pi*r/l
easy enough. I have performed this exact measurement to do the same thing. I had two identical antennas, and this worked just fine.
The distance and Tx frequency are not really related. Over long distance, they would technically attenuate differently, but inside a lab this is not appreciable.
Of course antenna gain is itself very dependent on wavelength. In general, higher frequency means higher gain for a given antenna (but only to a point; the antenna only has a certain range of lambas it can actually propogate)