Use the HM move Strength.
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You can use the move strength in Pokémon to move boulders. You can use strength in FireRed after getting the rainbow badge.
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.
With the HM move strength.