The Gardner River has three primary sections. The upper section (headwaters to Osprey Falls) is primarily a small brook trout fishery. The middle section (Osprey Falls to Boiling River) is mostly rainbows but there are good numbers of browns and cutthroat as well. Fish in this section range up to 15" with occasional larger fish. The lower section (Boiling River to the Yellowstone River) is interesting because of the warm water that enters from the Boiling River. It makes this section very fishable in the early and late parts of the season but it is really too warm to fish in the hottest months of the summer.
My few encounters with this river were in the warmer months and terrestrials were the key.
Gardner River | Fishing - by Big Sky Fishing.com
DIY Guide to Fly Fishing Gardner River in Yellowstone National Park - by DIYflyfishing.com
Gardner River - by Montana Angler