It could be nice if some technology would allow gatherers to focus on certain type of resource.
For instance, once you have cooking and hunting/fishing, you often rely on them for food. But you still need your gatherers, only for the water.
Later on, you have wells, so you don't need them for water. But they still bring back some, even if you would like them to focus on herbs or branches...
My sugg: have a "focus on" option on a gatherer (need some technology, and probably some tools too) : water, food, other material, or you could be even more precise with the appropriate research.
The gatherer would still bring back various items (after all, a gatherer could not resist the nice mushroom he/she saw on the side, just here, even if he/she has been told to bring back water), but would focus on the desired resource.