The term "gas giant" was coined in 1952 by the science fiction writer James Blish. It was originally used to refer to all giant planets. Arguably it is something of a misnomer, because for most of thire volume the pressure is so high that matter is not in gaseous form. Other than the upper layers of the atmosphere, all matter is likely beyond the critical point, where there is no distinction between liquids and gases. "Fluid planet" would be a more accurate term. Jupiter also has metallic hydrogen near its center, but much of its volume is hydrogen, helium, and traces of other gases above their critical points. The observable atmospheres of all these planets (at less than unit optical depth) are quite thin compared to their radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous (in contrast to Mars and Earth, which have gaseous atmospheres through which the crust can be seen).