The link OS::Neutron::Router is for the creation of the router. If this is not going to be router that has a gateway, that data is optional.

external_gateway_info External network gateway configuration for a router. Can be updated without replacement. Optional property.

If you take a look at OS::Neutron::RouterInterface You will see how to attach the interface to you network.

HeatTemplateFormatVersion: '2012-12-12'
...
Resources:
  ...
  TheResource:
    Type: OS::Neutron::RouterInterface
    Properties:
      port: String
      router: String
      subnet: String

In the above example you are providing the router UUID or name that you have already created, as well as a port and subnet that have been created.

In heat you are building you infrastructure in very small precise parts. Hope this helps. -JD