I don't necessarily know about "gymnastics" but I know we take LO to Rolly Pollies and obv. he is still in diapers. They start at 6 months there, but they have a class for every age group. My 2 year old nephew goes to Gymboree for their Gym class and he isn't potty trained at all either.
My daughter is involved with Rebounders (in Timonium). They come to her school once a week. DD started the classes when she moved into the older infant room, at about 8 months, so the kids definitely are not potty trained. I don't know about the classes you take at their regular location.
Re: Gymnastics for Toddlers