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.
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Well you need to get the TM strength from somebody and beat the gym (I forget what one) and teach it to a Pokemon
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.
HM04-Strength
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)
Any Pokemon able to learn the HM strength can move boulders. To shatter rocks you need the HM Rock Break.