By using a complex combination of *several* layers.
The psd file where I've got the texture layers saved is round'bout 160 MB and has five layers at 4096*4096 res. That's medum-sized compared to some of the habitable planets, though... :
The polar coordinates distortion filter is extremely useful, and it was used to practically every layer. It's a tricky beast though... you have to make upper part, lower part and middle part of a layer and then blend them together as smoothly as you can, in order to make a texture that can be wrapped to a ball seamlessly. Now keep in mind that this must be done to every layer before blending them together to form the end result with different layer modes.
The base colour layer is basically an uniform noise cloud layer which has been polar distorted and coloured to my liking.
The belts and zones are made by making a 16*4096 layer, filling it with uniform noise, then maximizing contrast and applying motion blur horizontally to make the dark and light areas horizontal; then I copied the layer, turned it 180 degrees and superimposed it on top of the original to make the northern and southern hemispheres more similar... Then I simply scaled that layer to 4096*4096 and there I had the basic colour variations for belts and zones.
The turbulence was the trickiest part to get right by far.
First, there's two levels of turbulence. There's lower level turbulence that doesn't care about the belts and zones and runs beneath them as strong base currents. Then there's the finer detail closer to the surface, which is mostly visible at the edges of belts and zones as mixing colours, light and dark...
So, I first had to create lower level and higher level turbulence maps and polar distort them both.
Then I took the fine detail layer, put the belt/zone stripe layer on top of it and figured out how on earth I could make it most visible only at the edges of belts and zones... Then I remembered the way to find the edges on images. So I did some edge stuff to the stripe layer and in the end I had a new layer, which had black on the edge areas and white on the belts/zones themselves. Then I simply turned black to transparent and lo behold, the fine detail layer formed, which had most detail visible at the edge areas and less in the middle of the belts/zones.
Then it took some creative mixing of layer modes to make the end result you see... Feel free to experiment, it's not easy and takes a whole lot of time especially on the first time when you have to figure everything out, but it's definitely rewarding.
After the texture is ready, I just mapped it to sphere and checked that it was distortion-free. It was.
Also, some gas giants have very little detail visible to bare eye. Saturn for example only has mild belts and zones, but it has the rings... and a load of moons. Also Uranus and Neptune have remarkably little detail compared to for example Jupiter. The amount of detail is almost directly proportionate to the temperatures on the planet. High temperatures usually mean big energy differences wich cause violent turbulence. If the planet is more uniform temperature, much less detail forms. Different strengths of detail layers can be used to produce different kind of detail, obviously.
Also, a lot of system RAM is recommended for operating with big files. I got fed up with my HD cache scrunching like no tomorrow when the page file got filled (! and I had like several GB's paging file on settings!) and slowing everything down, so I walked to a store and bought two 1024 MB DDR memory modules to add to the two 512MB modules I had previously... so now my system has 3 GB of DDR memory. Unsurprizingly the HD pagefile crunching ended immediately, and now I can swap between layers and big files much faster and without fearing that the GIMP might crash because of some error in the paging process.
It took a cut on my wallet, but sometimes you just have to bear the hunger for art...
I no longer wonder why graphics designers use such beastly computers as work stations.