The Virbank City Gym is designed like a music club. The 'puzzle' involves walking across a stage and battling trainers who are also band members. The air inside the Gym is filled with a purple haze, representing the Poison-type. You'll encounter two Gym Trainers before Roxie, who use Poison-type Pokémon like Koffing and Grimer. These battles are good practice for the main event and can help you gauge your team's readiness.
Recommended Pokémon: Ground-type Pokémon are super effective against Poison-types. If you caught a Sandile on Route 4 (later in the game, but good to know for future reference) or have a strong Psychic-type, they would be excellent choices. However, at this stage, Magnemite (Electric/Steel) from the Virbank Complex is a fantastic option due to its resistance to Poison and its Electric attacks being super effective against Flying-types (which Roxie's Whirlipede can evolve into). Even a well-leveled starter can do well, especially if it has a strong neutral attack. Bring some Antidotes to cure Poison status conditions.