How do you get the 8 Kanto gym badge in heart gold?

You don't get an 8th Kanto gym badge in HeartGold. There are only 8 gym badges in Kanto, just like in the original games.

Here's the order you need to beat them in:

1. Pewter City: Brock (Rock type)

2. Cerulean City: Misty (Water type)

3. Vermilion City: Lt. Surge (Electric type)

4. Celadon City: Erika (Grass type)

5. Fuchsia City: Koga (Poison type)

6. Saffron City: Sabrina (Psychic type)

7. Cinnabar Island: Blaine (Fire type)

8. Viridian City: Giovanni (Ground type)

Once you beat all eight gym leaders, you can challenge the Elite Four and then the Champion, to become the Kanto Champion!