The standard battery for the BS175/200 and BS350 is a 12N5-3B.
The 12N5.5-3B is ususlly slightly longer. It should fit the GTR, but when I tried one in my Dual twin it was just a bit too long to allow me to re-fit the frame side cover. Presumably the actual dimension varies slightly from different battery manufacturers, but I'l bet the 12N5.5-3B battery came close to the side cover on your bike - I guess you were lucky.
To be on the safe side, I'd suggest sticking with the smaller 12N5-3B on the 175cc and 200cc bikes.