The GSR2 by Thought Technologies is one of the most common GSR devices you are likely to find in the market. It has a computer mouse like appearance with the finger electrodes on the surface. It works by producing an audio tone proportional to skin resistance. Based on the number of these you find listed on EBay, there must be a lot of them out there.
The guts of the GSR2 are amazingly simple. I “reverse engineered” one to see how it worked. The manufacturer mischievously scratched off the label of the integrated circuit in a feeble effort to hide its value. After a little analysis I figured out that it is actually a CD4046 CMOS phase locked loop IC. With only a handful of other components, it makes a very successful GSR meter.
Below is basically the schematic diagram for the GSR2. I’ve left out a few things they did for an earphone connection. They are only using the Voltage Controlled Oscillator VCO part of the 4046 IC. By selecting the VCO center frequency (f0) to be around 30kHz, they get a lot of frequency variation around 400Hz with only a little change in voltage. That saves having a preamplifier to boost the relatively small GSR signal.