Hi Benjamin, in principle you could use duotone icons with the GBC: however they are not based on a .ttf font, so its technically different from the last free FontAwesome version we ship.
Duotone icons are simply svg tags , check in this codepen
https://codepen.io/fontawesome/pen/yLPEeyzthe sample includes some scripts from fontawesome.com, it should IMHO be possible to use this at least in a GBC customization.
Not sure if a singular icon can be exported to singular svg file ..I'm not an SVG expert.
Regards, Leo