I'd never go with just Gaussian blur for softening because it's a dumb filter, it doesn't know the good details from the bad ones. If you want an easy way to soften skin then use Filter->Blur->Surface blur followed up by minor masking.
Personally, for skin smoothing i prefer a more complex technique: duplicate layer, Invert, set blend mode to Vivid Light, use Gaussian Blur and raise until you start to see the pores and the like. You'll now have a ugly grayish image and what filter makes gray disappear? Filter->Other->High Pass which you set based on how smooth you want the skin to be. As the final adjustment mask out the effect in the essential details: eyes, mouth, etc.
