How do I query GTM to determine which IP's are configured for Global URL

The short answer is "no". The longer answer is that, in the relatively simple cases, you could follow the configuration elements, remove Virtual Server objects that are marked "down", then derive the answer. This could be done by retrieving the relationship between configuration elements via SNMP or iControl. This is, however, greatly complicated by things like Topology configurations and (even more profoundly) iRules. In fact, when one factors in iRules, even the BIG-IP doesn't "know" which answer it will give until a request is fully processed.