Here are the locations of 6 sages (you automatically find the seventh one)
Location: Route 18
How to find him: Use Surf on western-most part of Nuvema Town and you’ll find Rood at the southwestern point of Route 17 and 18.
TM Received: 32 – Double Team
Location: Relic Castle
How to find him: Find Ryoku at the deepest chamber of the castle where you meet Volcarona.
TM Received: 04 – Calm Mind
Location: Chargestone Cave
How to find him: Bronius can be located at the B2F of the cave. Upon talking to him, you will engage a fight with two Team Plasma Grunts.
TM Received: 69 – Rock Polish
Location: The Cold Storage
How to find him: Zinzolin is hidden in the container inside the Cold Storage
TM Received: 01 – Hone Claws
Location: The Dreamyard
How to find him: From Striaton City, head to the Dreamyard, and he should be near the stairs
TM Received: 75 – Sword Dance
Location: Route 14
How to find him: Use the HM, Waterfall in Route 14 to go on top of the waterfall.
TM Received: 08 – Bulk Up
After finding the 7 sages you basically won the game...beat the pokemon league again, beat Cynthia, catch the legendary's, and complete the pokedex...that's it really...do all the stuff basically...
And about finding ghetsis, i am sorry to say but you can't get him caught in the game. you do see him once before battling N, though i guess he gets arrested off screen.
HOPE I HELPED ^_^