G/T is Gain over Temperature (ie, noise temperature). At HF, where external noise is the limiting factor, antenna temperature is not very important. At VHF, it becomes more important but at UHF and above, the noise temperature of an antenna, coax, power combiner, etc. start limiting the weakest signal you can hear regardless of the gain of the antenna. Noise Figure is really a spin off of Noise Temperature.
The problem with a Rhombic for receiving is that since it uses a termination resistor, it will never be below about 300 degrees Kelvin (room temperature). When aimed at the horizon, this may be acceptable since you are seeing the earth anyway (which is 300K, of course), but when aimed at the moon, a satellite, etc., better performance is possible. The typical TVRO antenna, for example, has a Noise Temperature below 120 degrees Kelvin and many modern ones are 50-75K.
Thus, to transmit, the Rhombic would have merit but for receiving it's gain wouldn't necessarily improve an incoming station's signal to noise ratio and that's what we're really after.
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