Comment 13 for bug 1947391

While testing this on Usurri, enabling the option and restarting neutron-server was not sufficient to get the 'gateway_mtu' option added in 'ovn-nbctl list Logical_Router_port' for already deployed networks.

I also had to set the MTU on the network again to trigger an update:
openstack network set ext_net --mtu 1500