If Jupiter, Saturn, Uranus, and Neptune are all "Gas Giants", why are they perfect spheres and have gravity

Because they have a metallic core.

According to this site:

The four giant planets are comprised mostly of an outer layer of molecular hydrogen and helium and a much thicker layer of metallic hydrogen. However, each may have a small solid core as large as three to 20 Earth masses at their center.

They have gravity because all matter has gravity, even gas. The planets' gravity compresses the gas so that it is quite dense - it's thought there might be solid hydrogen deep within Jupiter. All that density means there's quite a lot of mass, and therefore, quite a lot of gravity.

The spherical shape is the natural one for an object that is held together by its own gravity, as the entire surface is all at the same gravitational potential. Anything that protrudes above the surface tends to get pulled down. The gas giants are not quite perfect spheres, though. They're slightly flattened because rotation makes them bulge at the equator. this is especially noticeable on Jupiter, which has a day of only 10 hours.

This is because gas has mass. Mass has gravity. Since gas giants are so large, they contain enough mass and gravity to have moons. The reason they are somewhat spherical is because that is the most stable form for a large massive object in space to take on. Gravity pulls everything to a center of mass causing a very spherical appearance. They are not perfect spheres they are a little flat due to the spin.

matahari is correct they do have metallic cores. One thumbs up from me.

What shape would you expect gravity to make a huge cloud of gas? A cube?

they are so dense that it creates a gravitational pull. which forms a sphere