How to display SAML assertion from GetAssertion

[nibiau at free.fr]

Hello,
I have configured the Shibboleth SP over apache2 .
I have configured the different exports to activate the GetAssertion url to get the SAML assertion.
The GetAssertion url is well usable and I can get a content from it.
But I don't understand how is encoded the message.
Indeed , for me the message should be only base 64 encoded but I tried to decodeBase64 the message but it doesn't work.
Moreover , in the HTTP header I can see : Content-Encoding: gzip

So, how to retrieve the SAM xml message ?

Tks a lot
Nicolas