Kanto's eighth Gym Leader in Heartgold/Soulsilver is a Trainer named Blue, who lives in Pallet Town and is Professor Oak's grandson. He is a versatile Pokemon user, meaning that he uses multiple types instead of other Gym Leaders, who specialize in one type.
In Blackthorn City.
Clair is in Blackthorn City, you can battle her straightaway
the potion is in the pharmacy in cianwood city
just defeat blue, the 8th gym leader
If you are talking about the 8th gym leader in Johto, you go to New Bark Town, surf east, and go to the Pokemon League. If it's the 8th gym leader in Kanto, go to Saffron City to receive a Hoenn starter from Steven Stone.
if you you meant after you beat her,go to the dragons den on the back of the gym.you need surf and whirlpool.you have to past the test the master tell you(clairs grandfather).
use a garchomp that you could trade from a sinnoh game (platinum, diamond, or pearl) i used a lvl 53 and kicked they're butts.
8th Gym Leader: Claire8th GYM Leader is Claire of Blackthorn City. She uses dragon Pokemon. Yes indeed Claire is the 8th gym leader in gold/silver/crystal and she uses dragon type Pokemon just like lance
u will find him on Cinnabar Island near the northern side. But he won't battle you until you beat Blaine over at Seafoam Islands. hen go talk to him and he will return to his gym where you can then battle him.
you must beat the 8th gym leader and she will grant you passage to dragons den
you need to beat the 8th gym leader in blackthorn city. glad to be of service.
from professor elm aft the 8th gym before lugia/ho oh