After you reach Cyllage City and get a bicycle there, you should go to the Gym to battle Grant.
About Cyllage City Gym
Grant and the other trainers in the gym specialize in Rock-type pokémon. Rock is weak to Fighting, Ground, Steel, Water, and Grass-type moves. Rock is strong against Normal, Flying, Poison, and Fire-type moves.
On the bottom level, you can go around the back to see a waterfall. You can watch the waterfall if you sit in one of the benches.
Climb to the Top
Go to the right of the guy near the gym entrance and step on the blue tiles. Then walk toward the wall and you will grab the rock climbing grips. Climb up and to the right to get to the upper level.
Go up the stairs and you will find Rising Star Didier. He has a level 21 Dwebble and a level 23 Relicanth.
Don't use the rock wall to the right of Didier because it goes to a dead end. Use the rock wall behind Didier. After you reach the other side, the path goes past a slope. If you go down the slope, you end up near the gym entrance. Instead, go past the slope.
If you climb the rock wall to the left of the slope, you find Hiker Craig, who sends out a level 24 Roggenrola.
Go back down the rock wall and go to the left. You will encounter Rising Star Manon. She has a level 22 Solrock and a level 22 Lunatone.
Climb up the rock walls after Rising Star Manon. You will reach Hiker Bernard. He has a level 21 Rhyhorn, a level 21 Nosepass, and a level 21 Onix.
Go up the narrow climbing wall near the hiker and you will reach Gym Leader Grant.