A Gi is the top half of a Martial Arts uniform. Since I'm not sure if that was a question about definition or a question about whether that was what he was talking about. Gi is pretty non-specific though... it can refer to the japanese style of an overlapping front (Al la Kuno), or it can refer to the chinese style for TaiChi or KungFu (Al la Ranma) that meets in the front and is secured with frogs.
Hakama are generally, to my knowledge, specifc to martial arts uniforms as well, or rather, traditional samurai "garb". They were basically a long pleated pair of pants that's purpose was to hide the foot movements of samurai so that their next move would not be known. They have, in modern times, been adopted as common wear in Japanese Martial arts styles such as Aikido, Kendo, etc. Ironically, those two styles wear them for the same reason... their tie to the Samurai, and usefulness when performing complex foot work and obscuring it at the same time.
Kimono are generally one piece. The shirt is generally worn under the Kimono, and really isn't what one would think of as a Gi. Gi are for the most part made of much "stiffer" stuff than what one would wear under a Kimono. There are exceptions (Chinese "gi" are often Silk or Satin), and non-throwing styles generally will wear lighter cotton Gi... but anything like Aikido or Judo usually wear a double knit, stiff and sturdy Gi that is ment for your partner to grab hold of and use for throwing, or to hold up to constant falling.
Sorry for the long post... I get a little carried away when discussing clothing and martial arts when it relates to japan and china. please, if any of this is wrong, don't hesitate to correct me.
