you use strength. you get strength on route 42. you can use it in the wild after you beat the goldenrod city gym.
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First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
Strength
To move boulders in Pokemon Gold you need a Pokemon with the HM move Strength. Selecting the Pokemon in the menu outside of battle should let you use it if you have beaten the Olivine City Gym.
Use the HM move Strength.
With the HM move strength.