I have seen this film in the late eighties, together with other Oshima's movies and I could collect only a vague impression of exoticism. This before I visited Japan, where I currently live.
I have seen it again last night and I can confirm that "Gishiki" portrays some of the most specific aspects of the Japanese culture. It is a movie deeply ingrained with the rebellion against traditional culture and family, which is typical of the late sixties-early seventies, not only in Japan, but also in Europe. The same can be said of the use of sexuality as a powerful device to offset the established values.
The powerful Sakurada clan is brought to ruin by the same force that keeps it together, the powerful grandfather. This happens in a sequence of rigidly choreographed family reunions, in occasions of funerals and weddings spanning several years following the end of the world. In this sense the world of the Sakuradas is so traditional that many scenes could be set in medieval Japan, with minor modifications in the dialogs and costumes. Ritual suicides and uncompassioned sex are recurring estranging events which follow and precede these ceremonies.
At the end what has been taken away from the protagonist is his very childhood, and hence his possibility to exist as a human being.