Open-loop gain of Op Amp at low supply voltage
First, your question is misguided for at least two reasons:

Gain of an op-amp is not well controlled. It might be dramatically different from device to device. Simulators won't tell you about this variation, they'll just give you the performance of a &quot;typical&quot; device.
Well-designed op-amp circuits will give good results as long as the gain is &quot;high enough&quot; and won't depend on the exact value of the gain.

You should not operate your op-amp with a lower supply voltage than its datasheet specifies. There is nothing guaranteed about its performance under these conditions.
You also shouldn't expect the simulation model to be accurate when the operating parameters (like supply voltage) are outside the specified ranges. So you can't trust that your simulation results are reflecting the real behavior of these op-amps.


Now, what is going on in your simulation?

Data handling from simulations of a class of Op Amp (LMC6482) in NI Multisim showed that open-loop gain drops drastically when a certain supply voltage is reached (from high to low voltages) as is expected, but it continues decreasing, reaching almost zero when supply voltage is not zero yet.

Probably, as the supply voltage drops, the input common mode range changes. At some point, the bias point you chose for your simulation went out of the common mode range for that supply voltage, and the amplifier gain dropped.
It's also possible that at some point the bias point of some internal nodes in the circuit get out of whack, breaking the connection from one stage to another. Again this could easily lead to a drop in gain.
As mentioned before, you shouldn't trust the simulation model to be accurate under these conditions --- the simulator is meant to tell you how the circuit will behave under typical operating conditions. It's very unlikely that the engineers (or interns) who developed the model optimized it or verified its accuracy for simulating operating conditions outside the datasheet limits.