It is on the Stormy Beach Route from the Pokewalker. You'll need 65,000 Watts and the National Dex to unlock it.
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.
Ash is not in Pokemon HeartGold, but Red is able to battle on the Summit of Mt. Silver.
No you can't
you dont.
You can't battle Steven in Pokemon HeartGold.
yes or no you choose
You can't battle Steven in Pokemon HeartGold.
y-y-yes
no. Pokemon battle revolution can only take Pokemon from diamond/pearl