What do you do after finding the first six sages on pokemon white?
You cannot find Ghestis. He disappeared after you battled him and N at the Pokemon League.
he only is in Pokemon platinum and Pokemon black and white in Pokemon platinum you see him in jubilife city, and I think maybe at Team Galactic HQ. in Pokemon black and white after you beat the elite four, he is looking for the seven sages. after you find all seven sages he'll talk to you and he'll arrest the 7 sages. you will never see him again after that.
After finding the six sages you basically completed the storyline of the game. Now you can do whatever you want! To name a few things: Collect all Pokémon Create the strongest Pokémon team Train your Pokémon to the highest level (100) Participate in Pokémon Musicals Trade with your friends Battle with others (local and/or online)