“The Wayne Gretzky of professional developers” is a hypothetical person who would be one of the best professional developers - when you say someone is the “(famous person’s name) of (group of people unrelated to the famous person)” this means they are to that group what the famous person is to their own group. In this case, as you know, Gretzky is one of the greatest hockey players - so the Wayne Gretzky of professional developers refers to one of the greatest professional developers.
The sentence is saying that not even someone as great at professional development as Wayne Gretzky is great at hockey would have been able to succeed due to all the “negative subcurrents” (not sure what this means, but it seems to be something that would prevent success), so the narrator feels relieved, knowing it wasn’t their fault the session failed - no one would have succeeded in this situation.