How to convert Arduino GSR values to microsiemens?
A Siemens (aka a Mho) is a unit of conductance, the inverse of resistance.

You have a measure of the voltage (A/D value) between the pads so you can find voltage. You'll also need to know the current between the pads.
Convert your A/D reading into voltage and use that and the current to find resistance (using Ohm's law), and the reciprocal of the resistance is in Siemens. 1 Siemens == 1,000,000 uSiemens.

https://en.wikipedia.org/wiki/Siemens_(unit)