Try onOne's free version of PhotoTools 2.6, there's an effect in there called "Model Skin," and that will get you started. This is, by the way, completely free and won't time out, plus you can upgrade to other products in the range at a discounted price.
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.
Would you mind elaborating a bit? Do you do this on a separate layer, a duplicate of the background layer, after applying a blur filter?
Also, in Lightroom, I can paint with the adjustment brush set to Clarity -100 for a nice soft skin look. What would be the closest equivalent in PS?
Same as before: duplicate the layer, blur, set blend mode to Lighten or Screen, then dial back the opacity to taste.
In Photoshop, Clarity -100 would be blurring a copy of the layer, adding a layer mask and painting in which parts you want blurry.