you can add the default GW in the interface configuration for the ASG. As I've read your description above, there is one ASG interface that holds a static public IP address. Go to this interface and add the Default GW option there.
Totally unrelated: If you use IPsec site-to-site tunnels, you can also get rid of the ASG, since the device is (at least with the Network subscription) also able to terminate IPSec tunnels.
you can add the default GW in the interface configuration for the ASG. As I've read your description above, there is one ASG interface that holds a static public IP address. Go to this interface and add the Default GW option there.
Totally unrelated: If you use IPsec site-to-site tunnels, you can also get rid of the ASG, since the device is (at least with the Network subscription) also able to terminate IPSec tunnels.