Gauss law states that the product of electric flux and permitivity of the medium thru any surface is equal to the charge enclosed by the surface. ie mathematically (epsilon)E.ds=dq or (epsilon)d(phi)=dq (phi)=flux thru the surface in oredr to apply gauss law any"closed" suface(often referred to as gaussian surface) can be used... but we generally prefer to use surfaces that hav a certain symmetry in order to enable easy calcuation.. if u can be more specific about the problem u are havin with the application of gauss theorem...i can probably help u out.
yes , that is right , but the law as stated by gauss uses the displacement vector because it is 1 medium independant(normal incidence only) 2 electric flux was historically defined in terms of the displacement vector only.