Measure S21 between 2 antennas at certain frequency using a Spectrum Analyzer
Yes, you can deduce an S21 (forward gain) from that by dividing P_RX / P_TX but I think that that number would not mean very much.

Move the RX and TX further apart: that S21 will change

Move the TX antenna (assuming non-isotropic radiation pattern): that S21 will change

Move the RX antenna (assuming non-isotropic radiation pattern): that S21 will change

Stand in between RX and TX: that S21 will change

Place something conductive between RX and TX: that S21 will change

So I do not see how the S21 you'd get would mean anything at all. It would be the S21 under those circumstances. I fail to see how that number can be useful in any way.