I dont have a GSR, but I did have a Savage Scout, and used an Aimpoint M2 on it for a little while. I just used one of Aimponts low rings and it worked fine. I didnt get a cowitness, but it really wasnt a big deal, the Aimpont ring is basically a Weaver type ring, and comes right off without much effort.
I cant imagine the Ruger is all that much different than the Savage and the low ring wouldnt work. If the Ruger rail is milspec, you could also use one of the AR type lever mounts, but it may be a tad high.