Much like the Cinnabar Island Gym, the Viridian City Gym shouldn't be all that difficult to defeat either. This Pokemon Gym is full of trainers that prefer Ground Type Pokemon, and, similar to the Fire Type Pokemon the trainers within the Cinnabar Island Gym used, Ground Types are extremely weak to Water Type Attacks. You should have captured many Water Type Pokemon of your way to and from Cinnabar Island, and they will come in handy yet again with the Viridian City Gym. As mentioned with defeating the Cinnabar Island Gym, there is also a Move Tutor in Fuchsia City that will teach your Pikachu Starter Pokemon a Water Type Attack of its own. A couple of Water Types, as well as your Starter Pokemon knowing its own Water Type Attack, should be all it takes to defeat this Pokemon Gym as well.