The Gallatin will still fish during the warmer hours of the day. Hoppers, beetles and ants will be your best bet. If you're lucky you could run into a few mayflies. Nymphs and streamers will work when the fish don't rise. Search around the park section for some of the slower pockets so you can get longer drifts. There is no need to fish here before noon if you want to catch fish on dry flies. The mountain water is cold already and will need time to warm up everyday.