I was looking into this too and after a google search I found a couple of helpful e-book previews (can't afford to buy the books
Handbook of Brewing edited by Hans Michael Eßlinger has a table on page 56-57.
Using a "standard malting regime" it found extract potential of quinoa at 83.2% or 1.038.
Amaranth was 79.7% or 1.037
Other GF grains are listed including a variety of millet (63.6%/1.029) and buckwheat (52.9%/1.024)
Andrew Lavery on AHB says millet is around 70%/1.032 (and reckons he gets close to 100% efficiency with a decoction mash).
I have a random excerpt from somewhere that lists buckwheat as 65.3%/1.030
Obviously there is variation with variety of grain used, and your malting process
Gluten-Free Cereal Products and Beverages edited by Elke Arendt, Fabio Dal Bello, mentions that a standard mashing regime can probably
be used with quinoa as it has lower gelatinisation temps than most other GF grains.
Both books note amaranth had poor attenuation, but I note one of these references only gave a week for the wort to ferment out.
MariposaSouth's answer is probably better as they have actually tried it. Same with the comment about Andrew Lavery and millet. Real world practice counts for more.