From what I remember there is an attribute that contains the XML used to filter. There is also a unique ID attribute. You need to create the Unique ID.
When you add the XML for the global collection you need to 'escape' the XML code so it works within the XML used to perform the XML post. I know python has some modules to do this. I'll try to find out what I did and post later this week but this was the most frustrating part (took me a while to figure this part out!!!)
Also I'm sure you have already worked this out but you need to create the GC on each landscape in a domain if you have more than one server!).
See:
REST API Global Collection creation issues
How to create a model within a static global collection via REST API?
Regards,
Frank