Yes, that's the right one, so you have that sky texture.

Check that this file is available to your Workshop 13 project - "Standard.wad" must be visible on a bar in the WED texture view, and must contain the sky_gorge texture. You can activate displaying the texture names by right clicking into the folder window. If it's not there, you've probably accidentally removed it from the project, or have copied an older standard.wad file into your workshop location, or are using another, older Gamestudio version somewhere on your PC.