It seems that you recorded the raw GSR values, which are are not scaled to the normal data range. There is ‘calibrated GSR' channel on the Shimmer element that you can add to the future recordings. It should contain GSR values in the normal range..
Regarding already collected data, you can manually convert the raw values into the normal GSR range. I think, it can be done with multiplication to a fixed factor, however, it can be better to contact Shimmer support for the exact formula.
Alternatively, you can run a test recording with both raw and calibrated GSR channels, record them, and then figure out a conversion formula by matching the corresponding raw and calibrated samples.