You cannot find Ghestis. He disappeared after you battled him and N at the Pokemon League.
The is nothing else to really do. The seven sages were basically the final mission.
i have been on several official cites looking for this including Pokemon wiki and serebii they list how to find the seven sages but not ghetesis so i think you cant sorry
First, there's seven sages. Their names are Ghetsis, Gorm, Bronius, Rood, Zinzolin, Giallo, and Ryoku.
After you have Looker arrest the last sage out of the Seven Sages, He should thank you and he should also tell about a boy who sounds exactly like N, who has been seen with a dragon pokemon somewhere far away... ^_^
You should have Zekrom already by that point. If you're playing Pokemon Black, then remember that Zekrom is N's pokemon, so you won't be able to get it. You need to trade it from Pokemon White.
Nothing that's it
N's location in the end is a mystery; after finding all of the Sages, Looker tells you that there was a report of someone seeing a dragon with a boy in a farawat region. What region, however, isn't mentioned.
nothing.
Gone forever =(
I think that's it and you just train your Pokemon up
Only if you beat the Pokemon league.
Sadly after months (minutes :D ) of research, I have finally accepted that Ghetsis cannot be found even after you find all the sages in Pokemon Black/White. There are rumours though that he has been spotted near Black City/White Forest, depending on which game you have.