Check your texture sizes. Some video cards demand power of 2 sizes e.g 512x512.
Once i had these gaps too. I found out that my textures were 511x511. The graphics hardware fills the last pixels with zero-data.
I changed the background color (used for clearing the color buffer) and i found out that the gaps had that color too; They were transparent (alpha = 0).

Offcourse i'm using D3D, but i still hope this can an answer to your question.

[offtopic]

What sizes do you use for your skybox textures??
I want to experiment with skyboxes too, and yours looks great so i'm curious about the texture sizes.

[/offtopic]