In the individual buffers, the normal buffer and depth buffer both show the whole mountain, and in the diffuse color buffer, the mountain appears partly cut off, leaving that empty space that you see.
Right before I draw to the GBuffer, I clear the screen to black, set DepthStencilState to Default and BlendState to opaque. Then, to prepare it with default values, I fill the color buffer with black pixels, the normal buffer with gray, and depth buffer with white. Additive, not alpha, blending is used for the lighting pass, but I don't see how that would have any effect since the GBuffer is already created at that stage.
UPDATE: I just realized what it was. I was going to post a screenshot showing the individual buffers and also removed the skybox from the scene for visual clarity. After removing the skybox, I realized that the colors were being rendered correctly and nothing was clipped off prematurely. It seems that, although I put a restriction to render the skybox in its own depth range, I still needed to scale the skybox by a reasonable amount, or else it would cut into the scenery. So I changed the scaling amount from 20 to 200 and the colors aren't cut off anymore! And the skybox still appears the same size due to perspective.