It can't be because of a collection of gasses as the sun.

Not entirely sure, but here's my theory.

A sphere is the most stable three dimensional geometric shape. If it wasn't completely round, with so much mass, it would collapse upon itself.
Have you ever built a lego tower as tall as you can? Even if it is straight, it will eventually fall. It needs support. A sphere gives the most support.

Plus, naturally, spheres are what fly around. Look at an atom, protons, neutrons, and electrons are all spheres.

Someone else may know for certain, or something else though.

Yep, that's correct HH. Also, you should know, toastybob, that planets were not all solid from the start. They too were composed of gases, which cooled down and solidified later on, giving the solid planets we know about today.

Take the Earth, you know that the circumference around the equator is slightly larger than the circumference joining the two poles. That's because the Earth rotates, and there is some 'centrifugal' force which caused that slight difference.

Yep. Like, if you get a squishy ball, tie it on a string, wind it up and let it spin. It will be elongated in the center.

Didn't think of that one! ;)

There's maybe something I can add to shed some light onto this. All matter in the universe seeks the lowest possible energy level. That's what we mean by stable.

In the case of a set of particles, the lowest possible configuration is that in which all of the particles are as close as possible to the centre of gravity. This describes a sphere.

It doesn't matter if they're solid, liquid, or gas. Over the timescale of billions of years, even solids shift significantly due to planetary processes.