Gathering
Gathering is the most basic method of "doing things" in Subsistence -- picking things up off the ground that spawn there; sometimes called "ground spawns". This requires no tool, and can even be done in pitch black. Items which can be gathered are either organics (eg plants and plant material), or inorganic (various crates and backpacks).
Items which can be gathered are found below.
Organic
All Season
Seasonal
† Apples can only be found inside various loot containers.
‡ Harvesting has a chance of returning the plant or the seed.
The only other harvestable plant are Mushrooms - but as those require special equipment to access, they don't fall under basic gathering.
For a full list, see Plants.
Inorganic
These are all found at any time of year, though the contents of the common containers will vary in different seasons.
Common
All other containers requires extra work to acquire and open, and hence doesn't fall under basic gathering.
For a full list, see Containers.