I have caught numerous Trout with either pattern. I believe the Crackleback is a more multi-dimensional pattern, however. The Griffith’s is almost always tied small (18-20) and the body is only peacock (an admittedly great material). The Crackleback can have any colored body with two to three strands of peacock pulled up over the body. Hence, you get a variety of body colors and the effectiveness of the peacock. Also, note the hackle is wrapped to allow more visibility of the body. If the Griffith’s Gnat is supposedly a midge cluster, the Crackleback can also be tied for this clump of midges and also pulled under to represent emergers, etc. The pattern was originally tied (Ed Story~FC Feathercraft of Missouri) as a river/stream pattern fished on the surface, drifting down until it went under and stripped back in darting movements to elicit a strike. Below is a how to link with the ususal extraordinary photo by Hans Weilenmann.
I live in the Houston, TX area where stocker rainbows are stocked in many lakes and ponds in our area every winter. I began using the Crackleback last season on one of the nearby small lakes. After experimenting with different colors, the go-to guaranteed color was yellow body amtron, with xtra-small copper wire wrap, yellow/black hackle, and dark grey or black wood duck flank feathers for shell and tail. I caught over 150 trout on that fly during the season. I tried different colors and different flies, but kept coming back to the yellow Crackleback. I tie it on a size 14 streamer hook. I wound up giving a number of the flies away to fellow flyfishers who weren’t doing so well. Many trout would hit the fly as it dropped on the water, before it was wet enough to slightly sink. After the fly gets wet, it sinks very slowly, so I strip it with quick short strips as with many streamer flies.