OK. i get it now. You've run into the typical toddler problem -- once they get even 5 minutes in them in the car they won't nap once you move them. It's just a normal part of their development. To be honest, you've been super lucky to have it last THIS long! LOL!
You're going to have to try and keep her from falling asleep in the car. Difficult, I know, but you're only option. Otherwise, you might have to change this school thing she's in so that she leaves a bit earlier so she's not so sleepy in the car on the way home. If she falls asleep, you're doomed to no additional nap. Otherwise, you're going to have to let her have a more full nap while in the car! Either way, not a great option.
Can you change this gymboree thing for awhile? I'd say in another 2-3 months her A time will increase enough that her nap won't fall so closely to your drive time.