Can someone explain kawasaki's motorcycle model letters, what they mean?

It doesn't mean anything in particular. Most street-only Kawasaki's equipped with a four stroke engine use the letter "Z" in the designation. This applies to everything from the little KZ200 to the huge KZ1300, including the current ZXR's. It is a tradition that goes all the way back to the the original Z-1 of 1973 and predates the "Zephyr" models by a couple of decades. In the past, when they also made street-only two-strokes, they were designated by the letter "H" as in H-1 500, H-2 750, KH100, KH125, KH250, KH400, etc up to KH750. These are not absolute rules, so there are exceptions. For example the GPz500 is called an EX500 in some countries and the KH250 was originally called an S1 250 and there was also on S2 350... GT may mean "Grand Tourisme", but probably not since it what used only on a couple of models and not on the big touring rigs. The letter "G" designates a shaft-driven touring model as in ZG1300 Voyager, ZG1200 Voyager XII, ZG1000 Concours (aka GTR1000 in some ...  more