What about trying JET’s suggestion Effect>Path>OffsetPath with a copy of the original gradient? If you place that copy behind the original gradient, only a small rim of it will be visible at all. As it is destined to be the trapping zone, you will switch on the overprint attribute for it. Therefore it will interact with black, so that any deviation compared to the original gradient’s size and/or position will hardly be noticeable.

Klaus