The equation provided by namhkim on that other topic is for a simple circuit comprising of a potential divider comprising of a reference resistor and the resistance of your finger.
The actual circuit of your sensor is much more complicated, and so that equation does not apply. (especially when there is a sensitivity control that can be set to any random/arbitrary position).
Grove - GSR_v1.2_SCH.pdf (24.0 KB)
Did you notice that it was namhkim's only ever post and that he posted it 2 years after the original question was asked?