I've never heard of anyone using REDIRECT binding for SAMLResponse (assertion). Is there a chance you are thinking of ARTIFACT binding? ARTIFACT binding would result in the browser making a GET request to the SP, however, the GET request will only contain a reference to the assertion (SAMLResponse) that the SP would need to retrieve directly from the IDP via a configured backchannel connection. ARTIFACT binding is old SAML 1.1 technology and is seldom used today.
Siteminder IDP supports only POST and ARTIFACT assertion bindings. I'm not aware of any way to work around this.