The GameView editor window clears the entire game view rect with the GameViewBackground style (dark gray with drop shadow) in OnGUI, which is called after Update in user's scripts where he calls camera.Render().

Aras: A proper solution would be to render game view into a render texture, always; and then draw that in the correct place. Would make it much easier to apply this "custom aspect ratio" thingy. However, would also require implementing MSAA for render textures.