I think it's about porting the device. I don't recall this one from being available in ath79. I think that reading through this topic: Porting guide ar71xx to ath79? - #36 by lucize would be more helpful.
@dtaht AirGateway is built with AR9331 and looks very much like other XW boards.
In such case, most of the work would be extracting the system configuration from board file in ar71xx, writing the device tree for ath79, and then porting over the image build configuration from ar71xx. Looks like 4h for experienced developer 
Cute little device, BTW.