Grove Multichannel gas sensor v2 calibration

Ok, I dug through Multichannel_Gas_GMXXX.h file and I found the function that outputs the Voltage:

inline float calcVol(uint32_t adc) {
    return (adc * 3.3) / GM_RESOLUTION;

Q1. Is this the Vs voltage?
Q2. What is adc? It is not defined anywhere.

Q3. If the output voltage is Vs, then,

Rs = (Vc-Vs)/Vs, where Vc = 3.3 V. Is this correct? (is it like in MQ9 sensor?)

From here I can find my way and recalibrate everything. Just need some directions and validation.

Thanks.

V.