You have to know the total loss resistance (including skin effect, losses in variable capacitors) and the radiation resistance. The radiation resistance is easily calculated from the size (at least for a single turn loop).
The radiation resistance (well below 1 ohm at these frequencies and sizes) are in series with the loss resistance, so the losses (and hence gain) depends e.g. on the conductor size (10 mm copper tubing or some thin wire).
I whiled guess for a loop antenna is -10 .. -40 dB, while the gain for a typical ferrite antenna might be -60 dB or even worse. This should be compared to the huge atmospheric noise levels at those bands.