Making Gil Efficiently?

[Akuha]

Oh no no no, save that gear, because you can trade gear in like that, or the aetherial gear, for GC seals when you achieve the rank of Sergent second class.

I heard someone say something like that when I was doing a Copperbell run once, when they asked why I never pick up any aetherial gear (I don't because it's not tradeable, I consider it junk if I already have something better). After that I went to my GC's HQ and looked around, but never found anything resembling a gear trade in feature. I'll be sure to save up my seals for promotions and start doing that.