Gustafsson's deficiencies are different and less serious than Stanley's, IMO. Stanley's problem is that he struggles mightily pivoting, moving side to side, turning. Basically he lacks quickness. The is critical for a defenseman, particularly one that does not necessarily position himself very well like Stanley. NHL level forwards would expose this hard. Foot speed is not the easiest thing to improve. It can happen though.
For Gustafsson's problems were more in the an awkward skating stride. He was choppy. This is easier to correct with proper skating coaches, IMO. Speed is more his weakness, which is less neccessary as a forward than quickness for a D, IMO. He also brings far greater skills to the table outside of skating. Which is an important distinction. He has worked on it as well to it's not really an issue. There is skating which is easy to improve like Gustafsson, there is skating that is hard to improve like Stanley (stride v foot speed).
I find overall skating to be overrated as a attribute, particularly if you can bring other things the table. This is particularly for forwards. D it's more critical to be able to skate. Still though poor skating can be overcome by elite skills elsewhere. Gustaffson has signs of that. Stanley does not have signs of elite skill elsewhere, just signs of good skill elsewhere. That does play a part. And of course while I find it overrated, it is obviously important as the game is played on ice.