On the top toolbar (where File, edit, view, etc. are) there's a menu that reads "effects". Under that menu, there should be another called 'Blurs' and 'Gaussian Blur' should be in there. It comes with the software.
Most of the plugins that you'll use or install will be under a category in the 'effects' menu or they'll be in the 'adjustments' menu.
FYI, Effects are categorized based on what they do. for example, the distort menu has effects that distort the image and the 'color' menu has effects that will do something to the colors of an image. Effects also only apply to your selected area. If nothing is selected, it will just act as if the entire area of the canvas is your selection.