We could not remove the gateway configuration for interface G2/0/0 of an USG6600 in order to add just a default route.
First, we tried to remove the interface for all the policies, but the issue still remain.
Given the below configuration,
[fw-int2-GigabitEthernet2/0/0] display this
ip address x.x.139.53 255.255.255.240
vrrp vrid 37 virtual-ip x.x.139.54 standby
vrrp virtual-mac enable
hrp track standby
undo service-manage enable
bandwidth ingress 10000
bandwidth egress 10000
[fw-int2-GigabitEthernet2/0/0] undo gateway
Info: The interface has been referenced by a policy, so you cannot remove or modify the gateway
After a remote sesion, we found that the root cause for this behavior was the fact that interface G2/0/0 was added to a multi-interface configuration, which is used in policy based routing.
However, there were no actual policy-based routing configurations on the Standby device, neither on the Master device. So the multi-interface configuration was not in effect and could be removed.
After we removed G2/0/0 from the multi-interface, we were able to clear the gateway configuration and add a default route instead.