Saturn's rings are mainly made of water ice, with small amounts of "impurities" (dust or rock fragments) mixed in. We can tell that by the fact that certain colors in the near infrared, colors that are absorbed (taken removed from sunlight as it bounces off the ring particles) by water ice, are missing from the sunlight reflected by the rings.

Thank you for your interest in the Cassini Mission and the planet Saturn. The answer to your question lies more in symantics than in physics. The term "gas giants" is actually a misnomer. In reality, astronomy texts refer to the giant planets (Jupiter, Saturn, Uranus, and Neptune) as the giant planets or occassionally the liquid giants. It is true that only their very outer layers are gas and below that outer layer, the planet becomes liquid due to pressure. The term "gas giant" is merely a common phrase that has become synonymous with the large outer planets.