What is at the core of the gas giants?
There are several different types of planet which are known as gas giants. There are only theories about what there cores are like.
Jupiter and Saturn are true gas giants. They are composed of mainly hydrogen and helium.
Uranus and Neptune also contain large amounts of hydrogen and helium. The also contain hydrogen and helium. They also contain other substances including water, ammonia and methane. These substance are known as ices So, technically Uranus and Neptune are not gas giants they are ice giants.
We can theorise the composition of gas giants using computer models. These models try to explain hot planets were formed and the internal structures. It has long been thought that gas giants have rocky cores which accumulated gasses to for the planet. Recent theories suggest that this is not the case.
The internal regions of gas giants are under huge pressure. It is thought that Jupiter's core area is at a pressure of over 3,000GPa and has temperatures over 35,000K. At these pressures hydrogen becomes metallic. As much as 78% of Jupiter is metallic hydrogen.
Recent theories suggest that Jupiter condensed quickly from gases and doesn't have a rocky core. Other theories are that if Jupiter had a rocky core it would have been eroded away by the metallic hydrogen surrounding it.
So, gas giants may have rocky cores. They may have no cores as such at all. Their centres being metallic hydrogen or ices under high pressures. We need to take highly precise gravitational measurements of Jupiter to determine this.