my dd8 (3rd) was in a similar place, refusing work, especially writing...if I had a dollar for every "We just need to motivate her", I could afford a fancy private school, lol!
I think the biggest thing, that for some reason no teacher we have had would go along with, is to minimize the times when he actually needs to move the pencil across the paper. Can he do writing with a keyboard or even dictate it?
You do need a proper assessment so the school can focus it's resources and probably provide an appropriate IEP...but, as I am learning, there is no "one size fits all" gifted program.
We are in the process of having a full evaluation because the school wouldn't give me anything else (due to high test scores), though with the Tourette's I suspect you will get further with your school...and I pulled my DD out and we are doing independent study (with optional class time once a week) while we figure out who she really is and what she really needs.
Common sense says it's easier to adjust the environment to fit the child, but our school was having NONE of that, so I'm making an environment that does...
Also, have you read the Misdiagnosis and Dual Diagnosis book?