Gathering matters for DoL retainers. HQ gear helps when it raises Gathering. It also helps when you upgrade their gear and want to convert their old stuff to materia, too.

I used to think that Perception mattered, too. It does, but it does not. It helps the retainers bring back HQ mats. But really, what good do those HQ mats do anyone in an era where you can HQ anything from all NQ mats anyway? Its not like too many people are willing to pay more for some HQ mats on the board rather than getting the cheaper NQ, they just buy the cheapest on the board.

GP is totally irrelevant for the DoL by the way. So, those accessories are nothing more than a vessel for the highest Gathering materia you can put on them, and then they just sit there never needing upgrading until the next tier of materia gets released.