I have a simpler method using Photoshop:

1. have 3 layers: 2 clear ones and the background layer should be solid black or a plain starfield.

2. on the middle layer, create a sphere effect by coloring a small area white and then create 'shadows' around it with progressivly darker gray when u go far from the white area then create a 'terminator' black for the 'night' side

3. then use gaussian blur at around 13 or so so that looks like a gray ball with light shining from behind 'you' to your 'left'

4. on the top layer just create small lines of different colors and alternate light and dark colors then use gaussian blur of around 8 to 13 or so depending if it's a large or 'small' gas giant

5. you can also use this method to create rocky worlds but instead of belts just make it a solid color then draw 'lines' in a darker shade of the same color...and and use gassuian blur of around 5 or so so that it doesn't blur too much

6. you should lower the transparancy of the top layer to about 60% so that the light/shadow of the middle layer shows up and voila, u got a planet in about 20 minutes (if that)...

You should get results like these:

Gas Giant 1:


Gas Giant 2:


Rocky Planet: