F5 GTM - Load balance between Non-LTM IP addresses (Physical Server)

Certainly it can. We do this in our "Configuring BIG-IP DNS (GTM)" course all the time. You should be able to add each non-LTM server as a "generic" type GSLB server in its respective data center, then manually add the virtual server addresses to the server (as opposed to auto-discover which you can use for a BIG-IP LTM system). From there, load balancing DNS queries is the same as if you were doing it to a virtual server on a BIG-IP LTM system. Monitoring is a slightly different issue but there are plenty of options there as well.