When Milestone LPR makes detections it sends analytics events to the XProtect VMS. We have a sample that shows how to extract these events. From there I guess you would know how to send the data to your gateway.
The Alarm and Events Viewer sample can do this (notice the LPR Events option)- https://doc.developer.milestonesys.com/html/index.html?base=samples/componentsamples/alarmeventviewer/readme.html&tree=tree_2.html
Other samples-