Hi Sachin,
when specifying a server object in GTM (as you did for your generic host type servers) you have the option to define a NAT address. This will be the external address and GTM will respond with the external and routable IP address upon incoming DNS requests matching a Wide IP definition.
Same definition has to be done for GTM as a server, especially if you plan to build a sync group with multiple GTM controllers in different datacenters.
Based on the data center assignments GTM and other server objects may belong to the same data center and in this case GTM can check the virtual services on the "local" servers directly.
Thanks, Stephan