That is certainly some ugly behaviour.
So let me just say how impressed I am that your DD has made it to L6, that is a huge achievment, you both have much to be proud of.
So back to the ugly stuff, sadly many gyms are full of this type of parent, and with it comes their children who are being trained to be just as awful. I think all you can do is continue to behave well. Be polite, civl and respectful and it will only reflect on how rude they are being. Hopefully they will feel badly enough to cease acting like babies.
Jealousy is always ugly in any form. Explaining to your DD is a good thing, for her to understand that they wish their girls were doing as well as she is will help her understand, though not empathise.
As NOG said they will get over it once the season begins as they will be so wrapped up in their DD's beating the other L5's that your DD won't matter.
I often find it amusing that parents do not realise that gym is an individual sport, girls advance at their own pace and they quit in their own time. Expecting that the gym path will be the same for every child is just ridiculous.
Good luck, and remember that you can always come here for some support, we are always happy to share in your joy.