Gain-Bandwidth Confusion

"Gain" in the equation: $$GBP = Gain \times BW$$ is a linear gain, not dB. You can't plug dB directly into that equation.

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Furthermore, bandwidth (in most cases) is defined as the frequency where the output drops by 3dB so it's already accounted for. It's part of the definition. You don't need to account for the -3dB a second time.

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20dB is the open loop gain of the op-amp at DC.

1MHz is the GBP which means that it is the frequency when the open-loop gain of the op-amp drops to one.

Notice that I did not mention 3dB anywhere. That's because is not part of the definitions of either of these things (sort of, as you can see from the above equation there is some relation)