The Suzuki GT250 was an evolution of the T250 Hustler. In Japan there was initially a GT250 model which was fundamentaly unchanged from the T250. When the GT250 was released to the world it had been modernized with a disc brake at the front and with a new Ram Air cylinder head. Other changes were harder to detect on the surface, other than one of the prettiest fuel tanks on a Suzuki being replaced by a "bread box." Why the changes to a successful machine, especially since performance was not enhanced......purely marketing.....Suzuki was keen to recoup it's investment into the ungainly GT triple series and the GT250 was rebadged and modified to have a family resemblance to the ram air triples. Couldn't have a superseded technoilogy ie air cooled twins performing better than the new kids on the block could we?
When the GT250 model was released (1971 in Japan, 1973 in Europe), it was still, a reliable if uncompetetive machine. It was as large as many 400cc machines and could not match the Yamaha RD250 for performance and handling. Despit this in many countries, among them the Great Britain, it was a best selling motorcycle until the mid seventies, possibly because it was a sweet predictable handler and still retained Suzuki's famous capacity for reliability and durability. The follow-up 250 to the GT250 was the X-& which was a different kettle of fish altogether.....quite skittish and fragile in comparison.
The T250 had a slightly more powerful engine than the GT250. Pollution and noise regulations of the tiome required Suzuki to redesign the exhausts which reduced performance a tad.