What parts of the message are compressed when gzip compression is enabled?

What parts of the message are compressed and what http headers are added when gzip compression is enabled on a soap request message?  I'm working with a 3rd party that requires gzip compression on the soap request message.  They provide little assistance but their documentation states packaged software like SoapUI allows users to simply enable HTTP transmission compression via a setting while custom solutions would require the appropriate code.  I'm trying to figure out what parts of the Soap message SoapUI compresses and what headers they add so I can modify our custom solution.