Simply go inside then go downstairs.
In the north of the city, west from the gym (next to the Name Rater's Tent)
go into the underground path go down the passage way and there will be a door go into it and there will be a room with three buttons in it
In GoldenRod, go to the underground passage, and go to the photo area (Top Right area) A Grunt will think your new, and hand you a uniform.
a tunnel
You get it from Lyra in the goldenrod underground.
The bf in the department store and in the underground hall thingy in goldenrod city
3,2,1
The Goldenrod Underground.
In Pokémon Gold, Silver, and Crystal, the key for the Goldenrod City Underground is found in the Goldenrod Department Store. You need to go to the store's basement, where you will encounter Team Rocket. Defeating the Team Rocket members will allow you to obtain the key, which you then use to access the underground area.
in the underground tunnel in goldenrod
Underground in Goldenrod City.
there is a grunt in the underground,you just need to look for him/her(i dont know what gender).