El3et, i was having problems with GUI too, i switched to work with guiTextures that works with transforms…
if you want to create a bg and this fills any aspect ratio, just create a guiTexture, set all pixel inset to 0 , width and height to 0 too, and set scale X and Y to 1, and there you go.
Label has a lot of parameters, being an element of GUISkin, like margins, paddings, alignment, etc, and some of them could be causing this unnatural behaviour.
When you simply want to display an image, use DrawTexture instead,
Also, I uploaded a few months ago on the wiki a short code that allows you to replace Rect with scaledRect, that auto-formats your dimensions to be resolution independent without messing with Screen.width / height.