As already mentioned, RenderTexture is one solution, though you sure need Unity Pro for that. Another solution is, given that I understand your question correctly, to use an additional camera that only renders the 3D object you want to show on your GUI by using a layer mask and then set this camera's viewport to wherever you want your 3D object to be on your GUI by using Camera.rect or Camera.pixelRect. This works perfectly, I did it a few times already.

I think its not possible. But why dont make a cube with the texture?

Or why dont make a screenshot of you 3d model and use it as texture?

Edit: Mabye its possible with RenderToTexture (Unity pro??)

Thank you for answer.

I give up and look for the another way.