I did just make a minor edit to the post for clarity. You don’t need to have gathering professions yourself to complete a gathering-based mission if you are given one, but having folks in your group that can gather will help as the the materials needed for these quests can be traded (but cannot be placed on the AH).
If you have a gathering profession, it might even be a worthwhile business opportunity to go into the nightmare and offer to sell your services to passerby who might not have a gathering profession themselves. 