To be honest, I am coming to the conclusion after much experimentation (and much sampling of commercial GF beers like Harvester and Green's) that variety is NOT the key. The more variety of grains you use, the less of any of them you'll taste, and the more the strong-flavored ones like sorghum will dominate. The best beers I have made, with the most prominent malty flavor, have been made with pale and crystal millet and nothing else. Unmalted millet is not even in the same ball-park. Adding other grains, like roasted oats, malted buckwheat, rice, etc., only diminished the malty flavor of the millet. The only thing is that lacing and head retention are still a bit low; I am presently experimenting with the addition of some flaked oats or flaked quinoa, as well as maltodextrin (much as I *hate* the idea of "cheating" with additives at this point).
Buckwheat malt I have found to contribute next to nothing in terms of flavor unless moderately-to-heavily roasted, and heavily-roasted it is quite nice for a dark beer. Quinoa malt I haven't tried yet and have some curiosity about, but the beers I've done with sprouted quinoa did not impress me too much. It's also much more expensive than millet and I have a hard time imagining it's worth it.
Chestnuts I have found to taste like, well, chestnuts. I've had just about every beer Harvester has ever made, and done a few batches myself with chestnuts, and while it is a nice flavor, it is NOT in any way comparable to barley. It does help out sorghum beers quite a lot, in terms of making them taste *better*, but it does not make them taste like *barley*. It makes them taste like *chestnuts*. And chestnuts taste better than sorghum, IMO.