The Prevent server will process the traffic that is sent from the Web proxy. If the proxy is configured to send GET requests as well as POSTs then the Prevent server will analyze both.
A POST is usually more useful to analyze as it contains information being submitted to the outside world. A GET request usually contains information being 'pulled' down from the outside.
In order to ensure that the Prevent Server is only analyzing POSTs, configure the Web proxy integrated with the DLP Prevent server to not send 'GET' requests.