Well you need to get the TM strength from somebody and beat the gym (I forget what one) and teach it to a Pokemon
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First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
You will need HM strength, which you can get in Rusturf Tunnel. To use it though you will need to beat Flannery in Lavaridge Town.
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.