DNS load balancing to backend servers using GTM/LTM.
Speaking of GTM without the use of the LTM, you can load balance requests to a pool of DNS servers by creating a listener and assigning a pool to that listener.
Note (load balancing to DNS servers can be achieved by creating a pool from DNS > deliver > load balancing> pool)
the listener on the GTM is different than the virtual server on the LTM, you can check the listener configuration below:
you can notice that there is a DNS profile assigned to the listener. after creating this listener, if you navigate to the LTM > virtual servers, you can find the listener listed as a virtual server (but having a DNS profile assigned), if you tried to remove this profile and then navigated back to DNS > listeners, you will not find the listener anymore.
In conclusion, the GTM listener is used for DNS requests and you can load balance DNS requests across different backend servers inside a GTM pool. The GTM pool can have its own monitor same as LTM.
regarding the part about "by passing LTM", I think the LTM will be hosting the services. When the user receives a DNS response, the client will initiate a new connection (HTTPs for example) and start communicating with the IP that was returned from the GTM, which I believe it will be hosted on the LTM.
There is a difference between the connection opened with the GTM (just to receive a DNS response) and the connection which is opened with the LTM (service connection) for example: retrieving a home page for any of your services.
There are cases, where both LTM and GTM are hosted on the same BIG-IP, so the client will initiate another connection with the same F5 to open a connection with the service.
One last thing, regarding the delegation:
"test.facebook.com > xyz.facebook.com/abc.facebook.com/efg.facebook.com"
You can check the below article for delegating records using zone runner:
If i didn't answer your main request, please clarify it further.
As Mohamed_Salah_ mentioned the DNS will give the F5 LTM VS IP address as a DNS resolution even if the F5 LTM is not local (not on the same box as the F5 DNS) this is not an issue and then after that the real connections will be load balanced by the LTM to the real servers.
What you are saying sounds like you want to use Generic Host object on the F5 DNS that will point directly to the real server IP address but this only done when there is no F5 LTM product that load balances traffic to the servers and you then may need prober pool to control which F5 devices will monitor the generic host etc. and it is complex.
I suggest to better read the F5 operations guides below as get an understanding of the F5 LTM and DNS/GTM modules: