External site gather interval is 1 hour by default, and is configurable 
Please see Content Delivery Service for reference.
Given this default/configuration would you still be looking for a way to trigger this externally?
In any case, if you did want to trigger site gathers, you can do so on the Root Server by making a GET request to the following URL:
https://<root server name/ip>:<bigfix port>/cgi-bin/bfenterprise/BESGatherMirrorNew.exe/-triggergatherdb?trigger