Good Morning everyone,
I would like to know if is possible to hide the sky like the terrain. I've tried the command HideSky but doesn't seems work. Does this command exist or do they still have to add it?

There is no HideSky command that I'm aware of, and no reason for one that I can think of! Just use an all black sky box!

A fully inside level,no terrain no sky needed .

Dave

ok, thanks for the advice, it was to earn more fps.

To earn some fps you can use a low resolution, black sky texture. More you will not get even with a hidesky command, because the engine still load the .x file and the textures when it starts GG.

High quality textures are the biggest performance eater in every engine

I can't imagine rendering a flat black image is much of a drain on resources.

Your biggest performance drain are in this order:
AI
Post effects (Bloom, particularly)
Draw distance (specifically camera distance).
Texture depth/quality

Your biggest performance tweaks therefore are:
Intelligently using AI (I use a manual lua based distance check to keep them extremely short range)
Disabling bloom entirely as well as all other post effects
Optimizing draw distance for your hardware (specifically using the camera distance slider).
Making sure occlusion is set to 100 (to ensure entities are properly being excluded from view, it generally results in a 5% performance boost by my tests).
Set terrain shader, entity shader, grass shader to low. Medium is generally about the same but if you're really struggling this will help as well.

Using these settings I can get a 2.8 ghz dual core intel chip running no 3d video card (my son's school pc) doing software emulation to around 12-20fps regularly.

There are other tweaks of course, but this should impact your regular day to day stuff the most.