How can a cube be a Gaussian surface? For verifying the gauss law in sphere we have same theta between E and A,  have symmetrical and uniform electric field. 
But when we talk about a cube than we get symmetrical electrical field but we do not get same the between E and A at every point.we also doesn't get uniform electrial field as in sphere. so how can we say a cube a Gaussian surface?
**In the sphere we had verified when we are getting above condition and can say a Gaussian surface  but how we can say a cube a Gaussian surface where where we are not getting these condition.*
 A: Wikipedia defines a Gaussian surface as:

A Gaussian surface ... is a closed surface in three-dimensional space through which the flux of a vector field is calculated; usually the gravitational field, the electric field, or magnetic field.

So a cube can be a Gaussian surface, a sphere can be a Gaussian surface, the surface of a tree can be a Gaussian surface! It is just the name given to a surface which Gauss' law will subsequently be used upon.
There is absolutely no condition that the electric field must be the same at every point on the surface, or that it must be parallel to the normal of the surface, so as you rightly point out, even though these are not the case for a cube, we can still use Gauss' law on a cube since it is ultimately still a closed surface.
Finally, if you were wondering why the flux through a cube the same as the flux through a sphere, I refer you to the following question:
Why is electric flux through a cube the same as electric flux through a spherical shell?
A: The flux through any surface surrounding a charge is constant, but it is only useful in finding the field if the field is constant and parallel to the area vector at every chosen segment of the surface.  For this purpose it is pretty much limited to uniform spheres, infinite cylinders, or infinite sheets of charge.
A: The entire point of talking about flux is that there are mathematical theorems making flux a convenient tool. Specifically, the divergence theorem links what’s inside a surface with what happens on that surface, and the Kelvin–Stokes theorem links properties of surfaces with those of their boundaries. These theorems hold for any fields and any surfaces.
For the electromagnetic field specifically, you often want to calculate things such as the EMF induced in a circuit in the presence of moving charges, and while this can be done from first principles, it’s just more convenient to define a surface of a shape that yields particularly easily to calculations (any surface that has the circuit as the boundary will do), taking advantage of the fact that as a consequence of Maxwell’s equations, the relationships between parameters of electromagnetic fields have just the right properties for the above theorems.
