In my opinion applying gelshield before primer and antifoul is a great way to add extra protection to a grp hull and reduce the risk of osmosis blisters in the future providing the grp within the hull is already very dry. If it is not dry then its need drying first for a long period or with osmosis heat patches and washing etc. However I think a 2015 Dufour yacht would have the better quality (isophthalic?) gelcoat which already provides considerable resistance to osmosis though, as already said, epoxy will make it even better.
If you are having the paint layers scraped off then that is the time to consider adding epoxy (like gelshield) but in my opinion I would not say it is essential.