Purchase a GS Ball: Once you have earned the Cascade Badge by defeating Misty in the Cerulean City Gym, you can head to the Goldenrod City Department Store. In the basement of the department store, you will find the Game Corner. Exchange 10,000 coins at the Coin Exchange Counter for a GS Ball.
Go to the Radio Tower: Head to the Radio Tower in Goldenrod City. It is located on the west side of the city, past the Pokeathlon Dome.
Enter the Underground Tunnels: At the Radio Tower, speak to Mr. Fuji, who will give you the Rainbow Pass upon showing him your GS Ball. After receiving the Rainbow Pass, you can enter the Underground Tunnels.
Find the Generator Part: Use the Rainbow Pass to enter the underground tunnel. Walk to the right until you reach the next room. In this room, you will find a Scientist with a Team Rocket Grunt. To obtain the generator part from the Scientist, you will need to defeat him in a battle.
Return the Part: With the Generator Part in hand, return to the Radio Tower. Speak to Mr. Fuji once more, and he will install the Generator Part into your PokeGear.
Use the Generator: After receiving your upgraded PokeGear, you will now have access to the "Generator" function. You can use this feature to generate electricity in certain areas of the game, such as the Dark Cave and the Power Plant, allowing you to progress further in the story.