Yeah, Sky Blue plains friend area.
Eventually, at the Wishcash's place, you will see Medicham and Ekans talking. Keep talking to them for those days, until both have disappeared, and Ekans is at the bulletin board.
Look for a rescue mission to save Medicham in Wish Cave, save her, and then you get a wish stone.
Afterwards, talk to them again for days, and then gengar will appear at your door.
Take him to Ninetailes, and then you'll get a new place. Take him to the end there, fill out this little test and then Gardevour will be free.
then, find her in the pokemon square, and she'll join your team.