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I implemented a custom function to not use the Sobel operation and emulate the gradient operation, it works:

pair<Mat,Mat> gradient(Mat & img, float spaceX, float spaceY) {

    Mat gradY = gradientY(img,spaceY);
    Mat gradX = gradientX(img,spaceX);
    pair<Mat,Mat> retValue(gradX,gradY);
    return retValue;
}

/// Internal method to get numerical gradient for x components. 
/// @param[in] mat Specify input matrix.
/// @param[in] spacing Specify input space.
static Mat gradientX(Mat & mat, float spacing) {
    Mat grad = Mat::zeros(mat.cols,mat.rows,CV_32F);

    /*  last row */
    int maxCols = mat.cols;
    int maxRows = mat.rows;

    /* get gradients in each border */
    /* first row */
    Mat col = (-mat.col(0) + mat.col(1))/(float)spacing;
    col.copyTo(grad(Rect(0,0,1,maxRows)));

    col = (-mat.col(maxCols-2) + mat.col(maxCols-1))/(float)spacing;
    col.copyTo(grad(Rect(maxCols-1,0,1,maxRows)));

    /* centered elements */
    Mat centeredMat = mat(Rect(0,0,maxCols-2,maxRows));
    Mat offsetMat = mat(Rect(2,0,maxCols-2,maxRows));
    Mat resultCenteredMat = (-centeredMat + offsetMat)/(((float)spacing)*2.0);

    resultCenteredMat.copyTo(grad(Rect(1,0,maxCols-2, maxRows)));
    return grad;
}

/// Internal method to get numerical gradient for y components. 
/// @param[in] mat Specify input matrix.
/// @param[in] spacing Specify input space.
static Mat gradientY(Mat & mat, float spacing) {
    Mat grad = Mat::zeros(mat.cols,mat.rows,CV_32F);

    /*  last row */
    const int maxCols = mat.cols;
    const int maxRows = mat.rows;

    /* get gradients in each border */
    /* first row */
    Mat row = (-mat.row(0) + mat.row(1))/(float)spacing;
    row.copyTo(grad(Rect(0,0,maxCols,1)));

    row = (-mat.row(maxRows-2) + mat.row(maxRows-1))/(float)spacing;
    row.copyTo(grad(Rect(0,maxRows-1,maxCols,1)));

    /* centered elements */
    Mat centeredMat = mat(Rect(0,0,maxCols,maxRows-2));
    Mat offsetMat = mat(Rect(0,2,maxCols,maxRows-2));
    Mat resultCenteredMat = (-centeredMat + offsetMat)/(((float)spacing)*2.0);

    resultCenteredMat.copyTo(grad(Rect(0,1,maxCols, maxRows-2)));
    return grad;
}