Hi,
I think that all answers to your question are in gaussianblur doc

|ksize|Gaussian kernel size. ksize.width and ksize.height can differ but they both must be positive and odd. Or, they can be zero’s and then they are computed from sigma.|
|sigmaX|Gaussian kernel standard deviation in X direction.|
|sigmaY|Gaussian kernel standard deviation in Y direction; if sigmaY is zero, it is set to be equal to sigmaX, if both sigmas are zeros, they are computed from ksize.width and ksize.height, respectively (see getGaussianKernel for details); to fully control the result regardless of possible future modifications of all this semantics, it is recommended to specify all of ksize, sigmaX, and sigmaY.|

You can read this tutorial too about smoothing images

Feel free to ask question about content of this document