That is not really a question to ask but if you asked who is ghetsis he is one of the seven sages in Pokemon black and white.
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Ghetsis should be in front of Black City for Pokemon Black.
In the dreamyard after you beat N and Ghetsis.
Both Colress and Ghetsis are villains in Pokemon Black 2 and White 2.
If you mean after you find the other sages . well you cant find him ever again you will have to wait for the sequel of pokemon black and white so really you cant find N or Ghetsis ever again in your currant pokemon game
If you mean battle him, you do that after you beat N in N's Castle.