I don't think you can battle Gardenia in Pokemon HeartGold.
let your Pokemon battle
No they can't.
No, you can't battle Maylene and/or Crasher Wake in Heartgold.
Battle frontier
She is in Cerulean City, in the Cerulean Cape. You will have to battle your way there however.
she is at cerulean cape
you must go to route 25 then go to right until u see a staircase go up and u see misty
misty will be a gym leader in HeartGold and SoulSilver
You must go and battle Misty. Once you defeat her, she will give you a badge. You have then defeated the gym.
Find a pikachu in the forest Train it until level 60-70 Teach it thunderbolt (or another electric type move) Shock Misty Easily
Where you find Misty.
Misty is somewhere on Route 25.
I don't think you can battle Gardenia in Pokemon HeartGold.
cerulean city kanto.hope it helps!
it is in misty's gym press a on the life savers
Travel to the end of Route 25 and she will be standing there.