You have my sympathies, because my DD has had similar. She's 10, does gym and trains 20 hours a week. She told me today, the teacher was grilling her about the amount of hours she trains.
I think teachers / others can be quick to blame everything on the gym, but they've never seen the child without doing the gym, so how can they judge? I know lots of people often comment about the hours my DDs train (DD2 does almost 8 hours, she's just 7), and automatically blame the training. However it's not necessarily that. My DD trains 3 x school days, so she has late nights, but she's always up ready for school the next morning. She just does have a lot more energy than the average child. Last year, she had several months out due to broken bones, but she didn't suddenly get more energy. I think sometimes this can be a bit of a red herring.
Regarding your child crying a lot, has your daughter always done this? If its new, and her hours have increased recently, then maybe it is down to tiredness, but it could also just be her personality. What did her previous teachers say she was like? Has your daughter said why she gets upset? I think a lot of gymnasts can be perfectionist types (from observation). I know my DD hates getting anything wrong, because at gym they train and train until something is perfect. But, that's not because of gym, it's always been her personality. Personally, I would try and work on the crying, that would be a concern for me.
Unfortunately, the hours spent in the gym will have an affect on her schoolwork. I can see it does on my DD. I think you have to accept that. My friends whose DDs have given up gym have all said about how their schoolwork has immediately improved. But, at the same time your DD is not behind. I know my DD would probably be so much better if she didn't have gym, but she's not behind, so I try not to worry. They get so much other stuff out of the gym: self discipline, organisation, maturity, team work, friends.... That I think it more than compensates for what they lose. I personally wouldn't worry about the extra work. So long as she is doing fine. Is your DD at a pushy school? My DD does the bare minimum of homework, but I'm a teacher, and I've read all the research which shows homework has little effect on improving performance, so I don't believe all that crap
.
Finally, some practical things that may help. On gym nights, my DDs pack their onesies and get changed straight into them after gym. They wear them home. This cuts down on faff time at home. I also prepare hot food before I pick them up from school, and leave it in food flasks, so they eat their supper in the car on the way home. This means they're not going to bed on a full stomach (we have a half hour drive home) and it lets the food digest a bit. When DD gets home, she goes straight to bed, but is allowed to read in bed for a bit if she needs to help her unwind and relax. To minimise the impact, I think you need to make the time after gym as efficient as possible. We are also very strict about early nights on non gym nights - for DD it's still 7:30pm. Most of her friends stay up to 8:30pm now.
I hope some of that makes sense. It is tough, and I know people can be very quick to judge (especially those out of the gym world), because they don't understand the hours upon hours of training that needs to go into learning how to do a giant on bars, or how to do a flick on beam. All my gym mum friends say the same. I'd be concerned about the crying, but if she's otherwise happy, I wouldn't be worried at all.
Hth.