Yes he is quite tough because his Pokemon know toxic. If you don't have a powerful Psychic Pokemon, he might cause some problems.
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The 5th gym is the Fuchia city gym Gym leader Koga
In Pokémon HeartGold, Janine is the new Gym Leader of the Fuchsia City Gym in order to replace Koga as the Gym Leader there due to Koga's promotion to the Elite 4, in addition to this, Janine is also Koga's daughter.
i'm pretty sure you get it from Koga when you beat him(Koga is the 6th gym leader)
The sixth gym is in Fuschia City, where you fight Koga, Poison type leader.
After defeating Koga the fuchsia gym leader he gives you the TM Toxic