Unfortunately, you can't change gears on bikes in HeartGold.
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You dont you can change gears using the B button.
there is no acro bike on Pokemon platinum, you press B to change gears
To change gears on the bike, all you have to do (while riding it) is press the B button. You don't have to hold it in, either. ~Lucario
In Pokemon HeartGold (And Pokemon SoulSilver) The bike road is on Route 17 in the Kanto region.
The B button. It's not even remotely hard to figure out.