It is very likely that all gas giant planets have at least a very small rocky or metallic core. All planets condense out of primordial nebulas that consist of gas, dust, and ice particles. The dust portion is composed of silicates and other minerals, and they have a tendency to clump together into planetesimals that gradually increase in size as their feeble gravitational pull collects material and increases it's mass. The larger they get, the more gas and dust they sweep up, until eventually they become full-sized planets. The silicates and metals will sink to the center of the planet due to their higher density, and that's what forms the core.
The only exception to this would be the first planets to form after the Big Bang, before there were stars to create the more complex elements in their cores. These primordial planets would consist of only hydrogen, helium, and perhaps a small amount of lithium.
There must be a layer of metallic hydrogen in Jupiter and Saturn; the temperatures and pressures require that it is there. There is almost certainly a small core of rocky material, perhaps some other metals such as iron, in the very centre of these worlds. The rocky core of Jupiter is probably more massive than the Earh, but much denser.
Uranus and Neptune, on the other hand, have layers of ices, water ice, ammonia ice, and methane ice, and also probably a rocky core.
See this informative image from NASA http://www.nasaimages.org/luna/servlet/ ... -Interiors