Actually what we see that when an electric field is uniform then we can calculate flux by the electric field lines going through the specified area but when an electric field is due to an electron then it matters hard to calculate by previous method because here the electric field is not in a specified direction (infact in al directons).So, we try to use gauss law here. Now the case comes of a cube put in an uniform electric field the flux through it is zero because the no.of electric field lines enetering in it also comes out from that.