Thank you Noros! You solved my problem! Googled it for an hour… Build 4.17.1 - the “problem” is still there. No additional includes in the header, the editor generation worked fine with that. Just an additional module in the *.Build.cs file.
Dear Sean, would be nice to have updated documentation at least. 
Adding “GameplayTasks” on this page in “Module” row (in the bottom). This would save hours for newbies like me.