You did not state whether her problem is with writing speed, actual letter formation, or clarity of thought. If the problem is speed, then have her pick a short reading passage she likes and write it down as fast as she can. Time her and make a game out of it. The next day, have her try to beat her time. After four or five days, pick a new passage. If her time improves, you may choose to give her a little reward, like a special trip to the library. If you are trying to improve speed, do not worry at first about legibility. You are just trying to strengthen the hand.
If letter formation is the issue, then practice is the only answer. Young gifted kids who are used to getting things quickly are not big on practice. Again, you may need to offer a small reward for effort in this area. Try interesting writing tools such as gel pens or a special notebook for writing to get her interest.
For both practice and improvement in idea formation try a writing session about three to four times per week. This is also a good summer idea. Here are some writing prompts that I have had non-writers respond to: "The best thing about today was..."; My favorite TV show is...because..."; "I liked the movie we saw because..." Little kids can draw a picture on the top half of the paper and write on lines on the bottom half. You can purchase paper that is already printed in this way, or you can print out some of your own.
Other writing games such as crossword puzzles, word-search books, and making your own newspaper, plus writing shopping lists for parents all give kids writing practice. Also remember that craft projects, building with small blocks such as Legos, playing a musical instrument, and so on, can build small muscle coordination which aids in writing skill. Try not to argue with her about her writing since it may actually discourage her interest. I suspect that right now her brain is "going faster" than her hand, and so she gives up on the writing. I hope you find this information helpful. Thanks for writing.