There was some discussion before on the topic of buddhist clothes
here in Dharma wheel. There have been explanations about the Hangesa/Wagesa on some sites, can't find them anymore, as usual! Because it is used by all or most japanese sects there probably are different explanations for it. And because of the presence of modern world buddhism it seems that they have adapted in Japan and have changed its explanation and history.
An older explanation is that the word Gesa comes from Kesa/Kashaya, and it derives from a laybuddhist's white robe, and it really is
the white robe, i.e. Kashaya of a layperson. They are in different colours, like white, golden and purple, the colours have symbolic meanings, gold for bodhisattva precepts, and purple is the colour for veteran or venerable laybuddhists. White is the traditional lay colour in indian Buddhism. http://en.wikipedia.org/wiki/Kasaya_(clothing)
White robes have been used also in tibetan buddhism by lay buddhists, in early Nyingma, by Jetsun Mila, etc... The development of lay Kasaya into Gesa or Wangesa has probably taken place in Japan. I don't know if there is anything similar in Korean or Chinese buddhism ?