This is very common - ability to read is different from the fine motor skills necessarily to write well (and having the stamina to write a lot). Add a lot of ideas rushing to get out of a gifted kids brain and you can see that many kids (gifted or not) may be advanced in reading and delayed (or only on grade level) in writing.
I wouldn't worry about it too much unless it was keeping a child from some form of advanced work (i.e. "no, your child can't go up a grade for LA because her writing is not at grade level"). If that was the case, you might do a bit of handwriting hothousing to bring her along (you can order handwriting without tears books online very easily). Or if you just want to support her writing (our schools don't do much in the way of penmenship) you can do the HWT or just encourage other things (as Mom of 1 suggests) that will develop fine motor skills.
