My original question was if there is a library that covers the internal comparator and gain modes for the Arduino Uno, Mega 2560, Leonardo, Due, Zero, M0.
The answer is obvious: "No". To my surprise there is not even a start to make it and neither a collaboration to gather usefull code.

If you know pieces of (Arduino compatible) code for the analog comparator and gain, then please tell me about it, and I put them in this list.

• analogComp, a library to manage the analog comparator.
  To use the analog comparator for the ATmega8, Arduino Uno, Leonardo, Mega 2560, ATtiny85, and others. A routine is called with an interrupt every time a particolar condition happens.
• Arduino Differential ADC and Gain ADC
  A file "wiring_differential.c" should be added to the Arduino files. Functions "analogReadDiff" and "analogReadGain" will be added. For the Arduino Leonardo and the Arduino Mega 2560.