She is a Sinnoh gym leader, the game is based off the regions of Johto and Kanto.
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I don't think you can battle Gardenia in Pokemon HeartGold.
There is no best Pokemon on Pokemon HeartGold.
there is no Pokemon Mansion in Heartgold.
it dissapears and you get a red scale but once you beat the elite 4 you get to challenge it
They are not in Pokemon HeartGold