Far from the Sun, where it was cooler, outer planets are able to retain most of its gas, which are now mostly liquid or frozen. Gases that were retained gave them even greater gravity, and collecting the remaining gas and dust in the disk faster than the inner planets grew even larger. Gas Giants are mainly composed of the original solar nebula material and they are too far from the Sun, because the gas and dust would evaporate in the hot inner solar system.
There is no clear dividing point between the surface and the atmosphere of gas giants. If one could sink deep enough into a Gas Giant, they may might find rock hard surfaces that are gases under intense cold and pressure. One may also find familiar rock and metal in the core.