Yeah, they're different dishes.
Souvlaki is meat grilled on a skewer, though you can get souvlaki pitas, where the meat is taken off the skewer and served in a pita, which is very similar to gyros pita except for the meat used. Kebab is the name for a whole load of different meat dishes originating from the Middle East. Often, it's used to mean döner kebab. The döner kebab uses the same kind of vertical rotisserie as the gyros meat, and the meat may be the same (though gyros meat is often pork, which you'll never find in a real kebab). There is a huge difference in filling, though, a gyros pita being almost defined as using
tzatziki (the "different sauce" Dimmie's referring to), onion and tomato, and often fried potatoes/french fries. Some minor variations may exist, but anything very different from that recipe is not a real gyros pita.
Kebabs vary much more in filling and use of sauce.
