GigaSpire GS4220E with Firewalla Gold

Hi.

I'm currently using a GigaSpire GS4220E modem/router supplied by my ISP.  I'm trying to determine the best way to hook up my Firewalla Gold. 

According to the documentation the best way to hook up would be to set the Firewalla in router mode and the GS as bridge mode.  It doesn't seem that the GS can be set in bridge mode, however.

Any suggestions on the best way to proceed?  The GS seems to be a good modem/router, so I was hoping to either use it as the modem or at least for WIFI, but not sure if that is possible if it can't be set to bridge mode...

Thanks