To decompress the gzip encoded response, you can use the filter stream wrapper provided in POCO.
In your header:
#include "Poco/InflatingStream.h"
Then, construct the Poco::InflatingInputStream with the std::istream and compression type:
std::istream &is = session.receiveResponse( res );
std::stringstream ss;
Poco::InflatingInputStream inflater(is, Poco::InflatingStreamBuf::STREAM_GZIP);
StreamCopier::copyStream( inflater, ss );
std::cout << ss.str() << std::endl;
...