There's many many tutorials on smoother skin for Gimp. You can google them or search them on YouTube. But as vT said, there's easier ways than the described one.
Anyway, almost every step on this tutorial can be done in Gimp as well.
1) Duplicate and desaturate (same in Gimp)
2) Filters > Blur > Surface Blur === Filters > Blur > Selective Gaussian Blur (different name in Gimp)
3) Make a layer mask and paste the middle layer onto the layer mask of the top one (

same in Gimp)
4) Image > Adjustments > Levels === Colors > Levels (different name in Gimp)
5) Make layer visible and play with opacity. (same in Gimp)
Photoshop doesn't have too many filters that Gimp does not have. Some have different names and different settings but that's about it mostly. (I do envy their brush dynamics though

)