Does anyone know how to add more skies in the skybank directory? I thought copying to that directory was anough but every time I tried to use some new textures I got a run-time error which make GG crash.

Yes there are a few things you need to prep and naming is the main key point.
The best thing to do is copy over a working skybox folder and rename it what you want it to be.
All the files inside that folder need to be the same name.
You will need the 6 cube textures in .dds format with the proper names of what they are.
open the x file in a text editor like notepad. It is in ascii format and is readable in notepad.
Find the 6 instances of the texture names and change them to your new texture name. Be aware of which side you are replacing, because the z file is set with the textures on the proper side of the cube.
Erase the .dbo file and restart Game Guru. This is a must or the new skybox will not work.
Test the sky in the test game editor and be sure the seems are matching up as they are support to.

You may need to do this a couple of times to get it right.

You are referring to GG Classic - yes?
If so check out one of the default skies in skybank to see what files are required.
And be sure of the basics:
1) The files must be in their own separate folder.
2) The folder must be named exactly what the .x file is named
3) You can name the textures anything you want, but the .x file needs to have those names for each tile in the set - you can open it in notepad.

hth

OldFlak....

Thank you guy, it works, but it seems I have to delete the .dbo file each time because if I go back to the test game mode, the sky becomes black except for the top surface. Also, the .x file seems not to be an ASCII file so how can I edit This one?

Download Note++ and edit the .x file.

Material Material__6 {
1.000000;1.000000;1.000000;1.000000;;
0.000000;
0.000000;0.000000;0.000000;;
0.000000;0.000000;0.000000;;

TextureFilename {
"clear_D.dds";
}
}

Material Material__2 {
1.000000;1.000000;1.000000;1.000000;;
0.000000;
0.000000;0.000000;0.000000;;
0.000000;0.000000;0.000000;;

TextureFilename {
"clear_R.dds";
... etc

Yeah, a GG requirement - Always delete the .dbo.

Whenever I update my models or skyboxes I always just delete the entire folder and copy the new one over.

OldFlak....

Edit the X file with fragmotion, it is free if you are a good prayer.

1-open X file on fragmotion
2- Export file as an X file
3 - choose text format

Now you can edit your X file on the notepad, and make the changes suggested above.

Not all the stock ones are encrypted, just find one that is readable and use that.

I posted one here but just be aware the back and front textures are swaped around in the X file.

OldFlak....