I'd be concerned for a number of reasons. The anxiety - sure, it won't be helping. But from my observation of difficult child 3, when he's anxious he loses his appetite. Why would she eat more? I'm not saying there's not a connection, but I would dig to try to find out why - for example did you, in the past, try to soothe her anxiety with food?
I agree about cutting out the Gatorade too. Down here in Australia it's hotter generally than a lot of places in the world, dryer and if anywhere we're likely to need to replace more than just water in summer, it's here. But the medical advice here is to drink WATER on hot days and NOT isotonic drinks. A sportsman who is sweating A LOT - and this has to be at the level of a marathon runner - would benefit from maybe every second drink being isotonic. But the trouble with isotonic drinks, is that they actually are not effective at replacing water, they're actually replacing electrolytes. And if you haven't lost electrolytes to the same amount that you're pouring them in, you'll actually increase the thirst (which you're quenching with more Gatorade... vicious circle).
We brought a rule in with our kids - every second drink has to be pure water. Putting ice in it is good, it helps you feel you're getting a bit of luxury, a bit of special attention. I like the long ice blocks that fit into a drink bottle. Alternatively, we freeze a drink bottle and then keep topping it up from a tap, so it gradually thaws and chills itself. If I'm going to the beach in summer, I'll grab a couple of drink bottles from the fridge, plus a larger bottle at room temperature.
As for some of the possible causes of weight gain/constant eating - there are psychological causes and there are physical causes. A cause I didn't want to mention, is Prader-Willi Syndrome. There is a boy with this in difficult child 3's drama class. It's tragic - it's a congenital problem in the hypothalamus where the satiety centre in the brain simply isn't working - these kids are convinced that they're as hungry as when they began to eat. Left unsupervised, they could literally eat until their stomach bursts. They simply cannot feel when they are full. To add insult to injury, they only burn 70% of the calories of a healthy kid the same age. They feel constantly tortured by hunger.
I also used to work for a professor who studied rats and eating. He was especially studying the pituitary gland, which sits right under the hypothalamus. Occasionally one of his rats (pituitary removed surgically) would be different - instead of staying slim and not ageing, it would eat and eat and eat... on autopsy (he would autopsy every rat, when it eventually died) he would find that during surgery the hypothalamus had been damaged. On closer examination, he found that only when one particular tiny area had been damaged, did these rats eat uncontrollably. This was similar to some kind of induced Prader-Willi. Different, because there are other aspects to Prader-Willi that involve other areas of the body, but it's a thought.
You say she was in a car accident at about the time the problems started. This could be psychological, or there could be a physical component. You need two different experts to give you some sort of feedback on this and you need a cooperative pediatrician to support your search for answers ("eat a stick of celery" - humph! Doesn't that bloke listen to you?)
I would be getting her seen by a clinical psychologist who works in cognitive behaviour therapy, to try to help her with the anxiety. And I would also be asking to have her referred to an endocrinologist, to assess the possibility that some injury from the accident could have affected her hormonally, at least with her satiety centre.
You need help. If she tests out OK with the endocrinologist then that will put the need for help back with the psychologist. Or the pediatrician may have other suggestions for help. But help you need, and not just celery. it's not magic about losing weight anyway. It's just another high-fibre vegetable... what about carrots? Tomatoes? Oranges? And yes, strawberries are fine (although can be expensive). Low in carbs, too. I seriously can eat a whole punnet, quite happily. Our favourite summer food is a big bowl of home-made fruit salad. No added sugar - it doesn't need it. Or you can freeze it, puree it and make ice-blocks with it - just as healthy, it just FEELS like you're being naughty. I would have a frozen fruit smoothie for breakfast - loved it. Until I began to head for low GI foods (which fruit on its own is not, unfortunately).
Good luck, this isn't an easy one.