How to Health monitor in GTM - Type BIG IP works?
The BIG-IP monitor (bigip) is designed to monitor a BIG-IP server defined to the BIG-IP DNS (GTM) system, and not for other types of servers. This monitor uses a special communication method called iQuery to monitor the BIG-IP server's status as well as the status of the virtual servers defined on it. It is always assigned as the monitor type when you configure a server and set the product type to BIG-IP.
Other non-BIG-IP servers can be monitored with other monitor types. I suggest starting with the recommendations contained here, then learning more about the different monitor types available for you to use on a BIG-IP DNS (GTM) system.
One last thought - although you can put content-specific monitors on BIG-IP DNS GSLB pool members, if those pool members are actually virtual servers on a BIG-IP LTM system and are already monitored on the BIG-IP LTM system, it's not necessary to have an additional monitor on the BIG-IP DNS system for the same resource. I recommend F5's Configuring BIG-IP DNS course for more information on monitoring.