You can not wear the gear your retainer has equipped. Its no longer in your inventory when your retainer has it. You can assign specific items to be gathered. Though its level based. Every 5 levels will grant new items, also not sure if you would to have gathered them also before it becomes available to your retainer. When I leveled my gatherers, except for fisher I would go and gather atleast one of each item on the list so my retainers always had a full list of items.