That is because gas giants and normal planets have different UV mapping. Normal planets have the cylindrical map with polar caps, the so called "cross pattern" - gas giants seem to have a standard spherical map or somesuch.
In other words, using cross-pattern textures for gas giant models results in the "messed up 3D puzzle" look. Or vice versa. You have to know the UV layout in order to do appropriate textures. Frankly, if you want to be a good texturer, you have to at least know the basics of modelling and UV-mapping. It makes it a hell of a lot easier to make textures that actually fit something. 
I suggest starting here. After that, you can also google for plentiful tutorials concerning your 3D package of choice.
If you, however, want to do it quick and dirty, the procedure for replacing normal planets with retextured gas giants is as follows:
1. Open normal planet .entity files and use their mesh entries to find out the .mesh file names which you will need to replace. Then replace those files with the .mesh file of the gas giants (renaming it appropriately, of course. You should not need to edit anything in the .entity files themselves)
2. Opening the new .mesh files, replace the texture entries with appropriate planet textures, taking care that they are properly formatted. As mentioned, the gas giants use (probably) a spherical UV-map, so you will need to have a texture which looks simmilar to that of the gas giants.
And that's it. Expect some heavy pinching and texture distortion on the polar caps though - that's why they used the cross pattern for normal planets.
