We consider the Gaussian curvature conjecture of a minimal graph \(S\) over the unit disk. First of all we reduce the general conjecture to the estimating the Gaussian curvature of some Scherk's type minimal surfaces over a quadrilateral inscribed in the unit disk containing the origin inside. As an application we improve so far the obtained upper estimates of  Gaussian curvature at the point above the center. Further we obtain an optimal estimate of the Gaussian curvature at the point \(\mathbf{w}\) over the center of the disk, provided \(\mathbf{w}\) satisfies certain ''symmetric'' conditions. The result extends a classical result of Finn and Osserman in 1964. In order to do so, we construct a certain family \(S^t\), \(t\in[t_\circ, \pi/2]\) of Scherk's type minimal graphs over the isosceles trapezoid inscribed in the unit disk. Then we compare the Gaussian curvature of the graph \(S\) with that of \(S^t\) at the point \(\mathbf{w}\) over the center of the disk.