What is the benefit of wrting OpAmp parameters as function of G and Gfc?
\$G_{fc}\$ is something like gain at the center frequency, or gain at the critical frequency, expressed in dB.

\$G\$ is the same gain, expressed as a voltage ratio (assuming equal input and load impedance).

It's not obvious why the other parameters are written in terms of G. Possibly it has to do with whatever your text is trying to teach. 


  Also, because the OpAmp open-loop gain is not large enough (just 56V/V)


If that's the device's open-loop gain, it should not be called an op-amp.

Are you sure it isn't 56 V / mV or 56 V / uV?