Does the API Gateway have an option to process the deflated and encoded SAML request?
The "Process SAML Authentication Request" assertion does not handle the SAML request if it is deflated and encoded.
The GZIP tactical assertion needs to be used for decoding and inflating the SAML request.
The GZIP assertion has option to deflate/inflate and base64 encode/decode. Please open a support case and request for GZIP tactical assertion that works with your gateway version.