Hi Bruno, I got this from
http://www.photoethnography.com/ClassicCameras/YashicaElectro35GS.html who got the information from the Yashica Guy:
"...
1966 Electro 35 Original flagship model
1968 Electro 35 G + changes in cosmetic design + ASA now 12-500 + gold contacts
1970 Electro 35 G (2nd ver) + revised film door ? Added "Color" designation to lens, but no change in design itself
1970 Electro 35 GS + cosmetic changes + ASA 25-1000
1973 Electro 35 GSN + flash hotshoe + minor changes to rewind lever
There are "T" versions of the above (GT, GTN). The T indicates a black top plate as opposed to a chrome one. (The "S" standing for "Silver Chrome or Satin Chrome")
...."
Not having a GS to compare to my GSN, I don't know what the "minor changes to the rewind lever" might entail exactly but it looks as though you're pretty much right in thinking the hot shoe is the only real difference between the two, although the GS does have a cold shoe and PC cable connection. Oh, and the GSN is mentioned as having a battery check light indicator which the GS doesn't seem to have. However, you might have better luck getting your hands on a GSN since they were in production for 17 years, as opposed to the GS's short run of just three.
That seems to be it, really, but I'm sure there's someone more knowledgeable out there who can correct me if I've missed anything.
Hope this helps!