background GUI texture

How can I get a GUI texture behind everything and use as a background?

I think you could set your main camera’s clear flags to Depth Only, then use a separate camera with lower depth, which would look directly at a plane with your texture on it.

Then, whenever your scene is rendered, the deepest camera will draw your texture, then your main camera will draw your scene. Anywhere that a skybox would normally be, instead there will be your texture.

See here.