Questions about science of brewing GF

There seem to be a lot of experts here about the different ingredients used in brewing GF and these ingredients all serve different purposes. I am trying to produce a Venn Diagram of sorts pertaining specifically to GF ingredients but don't have the working knowledge base to complete everything. Things that I am trying to understand are flavors and characteristics that different ingredients impart, as well as interchangeability. I.e. if sorghum is too tart can I sub a 1:1 of brown rice syrup. Why or why not and what changes? I know that this is lofty but I think it could be very helpful to new brewers like myself. Please feel free to add any discoveries that you have made and I will update my charts periodically and post to this thread. Thanks all!

Wow this is a big question. I'll answer part of it to get you on your way. Sorghum won't usually be too tart. It'll have some off flavors and that's why you might want to substitute brown rice syrup. If you were to substitute BRS for sorghum, you'd end up with a drier beer, i.e. a lower FG. The flavor would be more neutral and cleaner. So you could go 50/50 if you wanted to keep some of sorghum flavor but reduce the twang. Personally, I'll never use sorghum just because it tastes so strange to me. But that is aside the point. Another option is tapioca syrup. Briess makes one suitable for brewing. It makes for an extremely mild base to raise your OG. From there, it's easy to add toasted grains to achieve a certain flavor. Any malted GF grain is going to be fairly neutral without much malt characteristics, which is what most are going for in trying to replicate regular beer. Malted quinoa is pleasantly sweet and when roasted can produce most flavors you're looking for. I've found ~30 lovibond produces a nutty flavor. Roasted beyond that you'll get anything from chocolate to acrid, burnt. With rice, you'll get a variety of flavors. All rice can be roasted to various levels as desired. Red rice will produce a nutiness. As for the other rices, I have not experimented too much. Buckwheat can also be used as a base malt, though I'd warn against using 100% of it. Again, you can roast it and produce a grain that tastes akin to Grape Nuts, a cereal. If you've ever had buckwheat, it has a very distinct flavor, that of... well buckwheat. So if that's what you're looking for, then go for it. I can't comment on other grains as I don't have enough excperience. Another very useful tool in the arsenal of a homebrewer is buckwheat honey. If used in the right amount, it can bring a malt flavor that is hard to achieve otherwise. As for candi sugars and other sugars, it'd be the same as normal brewing so I won't go into that. Hopefully someone else can fill in the gaps.

One more thing to add. I forgot about oats and oat malt. Oats can be toasted and used for dark beers. They can also help with body and head retention. Thomas Fawcett and Golden Naked oats can convert if mashed for an extended period of time with amylase. They can provide a base for beers. The mashed oat malt tends to be fairly boring in flavor and would benefit from other roasted grains. Be careful with oat malt because it can be cross-contaminated and affect those very sensitive to gluten.

Also, http://glutenfreebrewing.wikispaces.com/file/view/Gluten_Free_Brewing___EB_2006.pdf

This is exactly what I was hoping for and gets me to a great starting point. I'm going to make a list of all of the GF bases and there possible uses along with flavors and characteristics. Thank you so much for the info.

Is there any reason that you couldn't just step mash, boil, and then add enzymes, rather than doing the decoction mash Andrew Lavery describes?

Why would you step-mash without adding enzymes first? That would accomplish nothing.

He doesn't seem to be adding enzymes at all ... or am I missing something from this:
http://glutenfreebrewing.wikispaces.com/file/view/Gluten_Free_Brewing___EB_2006.pdf

That's because he malts the grains, spurring them to create their own enzymes. It would seem a waste of effort to malt the grains if you're just going to add the enzymes.

I was under the impression that malting not only creates enzymes, but also changes the flavor profile of the grain.

Yes, it does...so I suppose in theory, if you were using malted grains, you could do what you described, but it seems a rather trivial step to decoct the enzyme-rich liquid and then re-add it. If you've already gone to the trouble to malt the grain, why bother adding exogenous enzymes when you can just decoct?