The Viridian Gym in its debut
The Viridian Gym battlefield in its debut
In the main series
The Viridian Gym was led by Giovanni, as it was in Generation I. The Gym features Roman architecture, stately pillars, and an old mural depicting the proud tradition of this most prestigious Gym. The predominant brownish color suggests the Gym's affiliation with the Ground-type.
The Viridian Gym first made an appearance in The Battle of the Badge. Before Ash could challenge him however, Gary Oak appeared, taunting him with ten Badges, and insisting on challenging the Viridian Gym for fun. Gary faced Giovanni, though his confidence was quickly eroded after Giovanni brought out Mewtwo, who defeated each of Gary's Pokémon, even when Giovanni allowed him to use more than one Pokémon at a time against Mewtwo.
Ash Ketchum challenged the Gym soon after, but after Gary and his Pokémon had recovered. However, as the Team Rocket boss had unmentioned business to attend to, he left the Gym in the care of none other than Jessie, James, and Meowth. Giovanni also left them with several Pokémon that they could use against any challengers. Ash had a hard time beating them initially as the new Gym Leaders had rigged the Gym to shock anyone whose Pokémon took damage. Eventually, Ash overcame the borrowed Pokémon's power and defeated all of them, causing Team Rocket to rely on their usual duo of Arbok and Weezing. Ash quickly dispatched the two, shocking the trio due to the Gym's rigging. They blasted off, dropping an Earth Badge in their daze. Ash caught it, but he, Gary, Gary's cheerleaders, Brock, and Misty had to escape the Gym quickly, as it was collapsing.
The Gym appeared again (presumably rebuilt) when Ash returned to Kanto from Hoenn in The Scheme Team, where Agatha was now the temporary Gym Leader. Ash challenged her to an exhibition match, but lost.
Giovanni abdicating his position as Gym Leader is similar to the case in the Generation I games; however, in the anime, he is still in charge of Team Rocket, while in the games he disbands the team.
The Viridian Gym in Pokémon Origins
The Viridian Gym battlefield in Pokémon Origins
In Pokémon Origins
The Viridian Gym was featured in the Pokémon Origins episode File 3 - Giovanni. Once Red arrived in Viridian City in order to challenge the Gym, he encountered Blue, who was revealed to have defeated the newly-returned Gym Leader just the day before.
Once Red entered the Gym, he right away noticed the different atmosphere it had compared to the other Gyms he had previously visited. The reason for this was revealed soon enough, as Red then found out that the Gym Leader was none other than the Team Rocket Boss, Giovanni. Red refused to believe that a man who treated Pokémon as tools for business, like Giovanni, could be the strongest Gym Leader of all. Giovanni, however, ignored this, simply stating that he didn't need his acceptance to make it true. As he asked whether Red wanted to battle him or not, Red replied that he was challenging him, but not as a Gym Leader, but as an enemy of all Pokémon. Seeing Red's arrogant attitude, Giovanni put away his usual Gym Leader team and took out two Ultra Balls, containing his strongest Pokémon. The first one was Rhyhorn, which, surprisingly, was able to defeat four of Red's Pokémon, despite two of them having a tremendous type advantage over it.
Eventually, Red's Hitmonlee was able to tie with the Spikes Pokémon, leaving both Trainers with one Pokémon remaining. Knowing that Red was going to use his Charizard, Giovanni revealed his other Pokémon being Rhydon, and said that Red made a mistake by saving Charizard as his last Pokémon. Red, however, told that he had intended it to be so right from the start, wanting to give the whole match a meaning by winning it with his Charizard. At first, Giovanni intended to crush Red's pride along with his Charizard, but when he sent out his starter Pokémon, Giovanni realized that the pose that he had taken and the way he threw his Poké Ball reminded him of his childhood, when he had also battled just for fun and excitement. After a climactic clash between Charizard and Rhydon, which ended with Charizard defeating the Drill Pokémon with Seismic Toss, Giovanni was ready to give the Earth Badge as a sign of his defeat, but Red refused to take a Badge from the Boss of Team Rocket. With his childhood passion for battling revitalized, Giovanni told his minions that Team Rocket was now disbanded, before turning back to Red and presenting the Earth Badge to him as the Viridian Gym Leader instead of as the Boss of Team Rocket.
Once Red left the Gym and headed for the Pokémon League, Giovanni decided that it was time for him to find a new path for himself and his Pokémon, indicating that, much like in the games, he was going to leave the Gym and train on his own.
The Viridian Gym was first visited in Bulbasaur, Come Home! in the Red, Green & Blue chapter, where Red and Professor Oak entered it, following a Bulbasaur escaped from Professor Oak's Laboratory. At the Gym, they are attacked by a wild Machoke, but Red managed to defeat it with Bulbasaur's Solar Beam.
The Viridian Gym made a brief appearance in at the end of A Hollow Victreebel and at the beginning of Make Way For Magmar, where citizens of Viridian City were asking questions about Giovanni.
Red revisited the Gym after the incident with Mewtwo. There he met Giovanni, who challenged him into a battle. During the battle, Giovanni's Nidoking's Earthquake caused the Gym to collapse. Eventually Red emerged victorious.
In the Gold, Silver & Crystal chapter, Red's Gym Leader test was held in the Gym. Even though he won rather easily, Red declined the position, due to still not having completely recovered from injuries inflicted on him by Lorelei during the Yellow chapter. Thus Blue was appointed as the Viridian Gym Leader instead.
In the FireRed & LeafGreen chapter, Yellow went to the Viridian Gym, looking for Blue, but only found his hologram, set there in case for a challenger coming while he was not present. An automated system then brought out Blue's Pokémon, which, even without their Trainer giving them commands, defeated Yellow's Chuchu in a battle easily. Soon Silver arrived at the site as well, looking for his roots.