Comment 2 for bug 1597324

The issue here is that your are trying to set the default gateway to an interface that is set to DHCP. MAAS in this case will not allow you because we have no way of saying this interface should have the default gateway since it will DHCP. To better understand think of the case where you had 2 interfaces that you wanted to DHCP, we have no way to ensure that the gateway from the DHCP server will be set as the default, either one of those interfaces could be set as the default gateway.