I haven't been to GSMNP yet, but its on my list and I have done alot of research on the fishing there. If its anything like Shenandoah, which I am assuming it is, the mountain brookies will eat just about anything, as long as you don't spook them and the fly is presented properly.
These guys are very knowledgeable on fishing in and around the park. I would give them a call or at least check out their website for reports.
R and R Fly Fishing | Fly Fishing the streams in Great Smoky Mountains National Park and the tailwater rivers of East Tennessee and western North Carolina