that's never been permitted. can either very carefully manually edit the config with viconfig and make sure you don't orphan any references, or backup the config and do the same edit and restore it.
thanks for feedback. I found out that if I rename the interface under INTERFACES from eg. OPT2 to WAN3 then the gateway in ROUTING is called WAN3. This is working if the interface is in DHCP or Static mode.
When it is in PPPoE the name is "OPT2_GW".