How do you get 7th gym badge in Pokemon heart gold?

Answer 1

Once you've conquered the first four gyms and obtained the Surf HM from the Ecruteak Dance Theater, head east of Ecruteak City. Surf your way over, and you'll find yourself in Mahogany Town, where the seventh Gym is located. In order to remove the person blocking the way, you will need to clear Team Rocket out of Mahogany.

To do this, first head north to the Lake of Rage, and battle the red Gyarados in the lake. When you come back to shore, Lance, the Dragon-Type trainer who was the leader of Generation I's Elite Four, will ask for your assistance in getting rid of Team Rocket. Head back to Mahogany Town and enter the house with a radio antenna disguised as a tree next to it; within, Lance will reveal the passage down into Team Rocket's hideout. Once the generator within is shut down, you will be able to enter the Mahogany Gym. The trainers within specialize in Ice-Type Pokémon, the levels of which will be somewhere from the mid-20s to the low 30s.