Forum Discussion
I believe that, because the LTM and associated virtual servers are using the bigip monitor, expected behavior is that the local GTM will fail all virtual server objects hosted on the LTM that it cannot reach via iquery (you should validate with F5 support) regardless of the other GTM's status.
If this isn't desired, you can use other monitors in addition to the bigip monitor that can operate in the manner that you described. For example, if you assign an additional HTTP monitor to a GTM virtual server object, the GTM will delegate that monitor duty to another F5 device in that virtual server's data center. If the GTM loses connectivity to the delegated monitoring F5 device, it can use another live F5 in that data center to monitor that virtual server object.