You need to have the HM4 Strength, obtained in the lost tower near Solacean town You need to have the 5th Gym badge, obtained in Hearthome City
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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.