Yes, as the others have said, no need to be nervous. But of course you will be, as this is your first time.

I would recommend sending an email to the gifted coordinator first, but since many people are not around over the vacation, if you don't get a response in a couple days, call the school and ask when the gifted coordinator returns or else communicate with the principal. On second thought, maybe send your first email to the GC and the principal, since the principal is the one to make placement decisions.
My advice is to ask for advice: "My kiddo recently tested 99th percentile on this test with this tester. Do you accept outside testing? It seems from what he is currently doing, he would fit in best with your GT program. If you don't accept this type of testing, is there any chance he could be tested with you before school starts? Thank you so much for your help. I am new to all of this and I'm really not sure exactly what to do"
Or something like that.... Good luck!