i just looked up abbreviated possible GSR interpretations.... there were MANY. but the most reasonable sounding Definition was "Grand Sport Racing" i bet that's what it stands for
The MR (top of the line) will have every available option and you really do not need to upgrade anything i.e. turbo, fmic, injectors and such. The GSR would be a tuners dream i.e. like upgrading the necessary hardware and parts to get the 1000whp everyone is dreaming aobut.
Springs, shocks, brakes, wheels, some turbo components, luxury components like leather, then some exterior differences like color of vents, grill etc. And last but not least, the TC-SST.