ok, i just did, went back to Hearthome city and no one's still there. Do i need to enter the pokemon contest or something? Because i didn't at all.
You should
Do i need to get in 1st place? I just went into the contest and i came in 4th place like 3 times...and there's still no gym leader.
No, just go in there and talk to your "mom".
Then head to The Mansion on @ bottom-left exit of the city