The main characters in a 'gym lover visual novel' might include a determined athlete who is training for a big competition at the gym. There could also be a novice who is intimidated by the gym environment at first but is inspired by the other characters. Another possible main character could be a gym owner who has his own story and relationships with the gym - lovers that come to his place.
Well, it's hard to say for sure. However, it's likely that there would be characters like a young woman who is into yoga and Pilates in the gym to keep her body flexible and toned. Then there could be a male character who is a powerlifter, always aiming to break his personal records. And perhaps a personal trainer character who helps other characters reach their fitness goals within the context of the visual novel.