The GS ones are shorter but I dont know by how much. I had a front go on my 1200 GSA recently and nearly bought a shock off a GS, I was told the front was shorter by an inch or so, but it would fit, dont know about the length of a rear GS shock.
I have a new spare rear GSA one I can measure if you like.
There are shocks from Wilbers and Öhlins that enabels lowering the GSA. I think that using the stock GS will not be a good solution, as the GSA shocks springs are heavy duty due to the bigger tank (more weight) of the GSA.
I just measured the OEM shocks I took off my '08 GSA.
Front: measuring from the centre of the bottom eye, to the top of the body, no washer = 350mm
Rear: measuring from centre of eye to centre of eye = 410mm
All we need now is someone else who has shocks taken off a normal GS ...