you can use the hm strength
you can smash rocks with rock smash
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You use Strength to roll the movable boulders.
First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
If the boulders are smooth then you need to get one of your Pokemon to learn strength to move it and if the boulder is rough then you need to get your Pokemon to learn rock smash ( both of these moves require HMs to be learnt)
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.