GZIP Compression in WCF Service

I'm trying to implement GZIP compression in an already created WCF service. I tried to use GZipStream class, to compress the SOAP response from the service. But when, I'm trying to test the response from Postman or SOAPUI, the response showing some non-readable format over there. I also included the header "Content-Encoding: deflate, gzip"
Is this normal?, How to verify whether the response is getting gzip compressed as per the industry standards