this should be all that is required to be set to "disable" the lxcbr0 interface and use only "some-bridge" that has a gateway

example-provider-networks-with-gateway.yml

global_overrides:
  management_bridge: "br-mgmt"
  provider_networks:
    - network:
        container_bridge: "br-mgmt"
        container_type: "veth"
        container_interface: "eth1"
        ip_from_q: "container"
        type: "raw"
        group_binds:
          - all_containers
          - hosts
        is_container_address: true
        is_ssh_address: true
        gateway: 192.168.1.1

static_only_containers.yml

lxc_net_gateway: 192.168.1.1
lxc_net_netmask: 255.255.255.0
lxc_net_address: 192.168.1.100
lxc_net_bridge: br-mgmt

lxc_container_default_interfaces: |
  # The loopback network interface
  auto lo

  # Load any additional configs
  source /etc/network/interfaces.d/*.cfg