Unlike previous Mainstream Pokemon Games, every Pokemon Gym found within this game have certain requirements that must be met before your trainer will be allowed to challenge said Gym. The third Pokemon Gym within the Kanto Region is found in Vermilion City. This Pokemon Gym is blocked by a number of trees, and because of this, you will be required to hop aboard the S.S. Anne beforehand in order to obtain the Chop Down Secret Technique for your Starter Pokemon. With this New technique learned, you will be able to chop down the trees outside of the Gym so you can gain access. No further requirement is needed after this.