string sourceFilte = "psdnet991_layers.psd";
string outputPsd = "out_psdnet991_layers.psd";
string outputPng = "out_psdnet991_layers.png";

    using (var image = (PsdImage)Image.Load(sourceFilte))
    {
        SmartObjectLayer smartLayer = (SmartObjectLayer)image.Layers[1];
        Layer maskLayer = image.Layers[2];
        Layer regularLayer = image.Layers[3];

        GaussianBlurSmartFilter gaussianBlur = new GaussianBlurSmartFilter();
        gaussianBlur.Radius = 10;

        // Apply filter to SmartObject
        gaussianBlur.Apply(smartLayer);
        smartLayer.SmartFilters.UpdateResourceValues();
        smartLayer.UpdateModifiedContent();

        // Apply filter to layer mask
        gaussianBlur.ApplyToMask(maskLayer);

        //Apply filter to layer
        gaussianBlur.Apply(regularLayer);

        image.Save(outputPsd);
        image.Save(outputPng, new PngOptions() { ColorType = PngColorType.TruecolorWithAlpha });
    }