I have been to east branch of the Swift river near Coos canyon, off Rt. 17, north of Rumford, Maine. A campground at Coos canyon caters to gold panners. I found a good little bit of color in the 3 times I went there, mostly from sniping for it. Bring several different tools with you to work cracks and crevices in the bedrock. Digging on the riverbank is not allowed, so you need to stay in the riverbed itself. Fall is a great time to go, when water levels are low.
Gold has been found in over 70 streams in Maine's western mts. These are glacial deposits, believed to come from the Chaudier river in Quebec, Canada.