Cree would work great on a Griffiths Gnat.
All the Cree I have is of the type that's got a ton of black mixed in. Its basically Grizzly compared to some of the really light brown Cree I've seen, so I use it interchangeably depending on the size I need. If it doesn't work, I don't reckon the feather is to blame.Just use Grizzly. The fish will not know the difference.