Happy to hear you made it through 
GW provides a very easy way to collect data from different systems,
all you need is to add system aliases to your service using the same transaction /IWFND/MAINT_SERVICE (on the lower right section)
then in the browser url you need to add ;mo (multi origins) after the service name
for example:
http://<host>:<port>/sap/opu/odata/IWWRK/WFSERVICE;mo/WorkflowTaskCollection
this will retrieve the tasks from all system configured , this is why you have default check box in the configuration to point out what system to use when mo; is not requested.
btw it's also nice to know that you can use ;o='<system_alias>' to get data just from a specific system, for example:
http://<host>:<port>/sap/opu/odata/IWWRK/WFSERVICE;o=SYS_100/WorkflowTaskCollection
When working with mo; you should also be aware that a new key is added to all entities,
so when doing a specific read it will know which system to address
you can see in the metadata of the service, a new key SAP__Origin is added everywhere.
http://<host>:<port>/sap/opu/odata/IWWRK/WFSERVICE;mo/$metadata
BR
Roy