Hi dushieet, I'm sorry for my late reply, but here I go:
The pedalSHIELD project auqio quality is limited by the Arduino (MEGA in this particular case) ADCs and DACs (analog-to digital converters and vice-versa). It is a fantastic project if you want to learn about how to code and build your own digital guitar effects.
The Zoom G1/G1 Four pedals include audio codecs ( that have better quality than the Arduino DAC/ADC) and also are programmed in a more complex/difficult coding language (DSP dedicated programming software). So simplifying, the answer is no, we cannot make the Arduino Mega into something like the G1.