It's similar to the new way desynth works. There is a base amount of gathering that has to be met, what's required to run the venture, and that gives you a base set of parameters for that particular one. As their gathering increases, the chances of getting more items or higher quality versions of said items increases. Be advised that for rare drops, like minions, though the chances of getting them might increase, it's not as significant as the quantity and quality of materials. In short, yes, upgrading your gathering retainers gear is beneficial, but you don't have to go out of your way to do it, so long as you reach the minimum gathering requirements to perform the venture. Giving them hand-me-downs is perfectly viable.