If you mean battle him, you do that after you beat N in N's Castle.
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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
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.
Ghetsis should be in front of Black City for Pokemon Black.
Well you defeat alder and he will register your pokemon on the hall of fame. After that you have to find all seven sages including ghetsis.
In the dreamyard after you beat N and Ghetsis.