The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm not sure how a griffith's gnat is supposed to imitate such a thing
Schwiebert's theory was that when it was awash in the film, the herl and halo of hackle suggested the loosening pupal shuck around the dark body of the emerging midge. Others have speculated that it imitates a cluster of midges. Both theories are reasonable, I suppose, depending on the size of the Griffith's Gnat that is effective relative to the size of the actual midge. Like many anglers, I just know that it does work. :)