Dysgraphia is a serious dysfunction in the ability to write fluently and coherently in a developmentally appropriate manner. It is diagnosed by a learning specialist using standardized tests and writing samples. An excellent source of information on this topic is the book Dysgraphia: The Writing Dilemma by Regina Richards. This book gives recommendations for teachers, parents, and students. In addition, visit the Council for Exceptional Children (CEC) website at www.cec.sped.org. The CEC provides fact sheets for parents to download on various learning disabilities. I suggest you investigate this informative site.
Gifted children understand only too well the problems and frustrations a learning disability brings. It is important that you help your little girl understand that this is not her fault, it is not occurring because she isn't trying hard enough, and that she isn't "stupid." (A sad word that LD kids have used to describe themselves to me.) This is just another way that her interesting brain works! With some self-help skills such as a use of a word processor, proofreading habits, and guidance from a learning specialist your daughter can make many gains in her writing ability.
I have only been able to give you some limited feedback here on the very large topic of dysgraphia, but if you look up the references I have suggested I believe you will find them helpful. Good luck.