What does the "FG" in the formal name of the Symphogear system stand for? The formal name of the Symphogear system is 「FG式回天特機装束」, which translates roughly as "FG-Type Sky-Returning Specialized Garment". (Maybe? I probably screwed something up, but this is close enough for the purposes of this question) 

What does the "FG" in there stand for?
In the manga translator's (http://symphogear.blogspot.com) commentary, the FG is guessed to stand for Fonic Gain (it should be Phonic Gain). However, I'd speculate that it would mean Phonic Gear as it is the more elegant and title-consistent explanation.
As we know, Symphogear could mean Symphonic Gear while Symphonic itself means (etymologically) "Sounding Together/Concomittant Sound" due to the syn- prefix means "together/with" or "concommitant"
 ref: