Gathering is one of the thirty-nine Avos Melachos because gathering was necessary for the construction of the Mishkan. Plants were gathered, after harvesting, to make dyes.

The definition of gathering is putting together scattered grains or produce, in one place, to prevent them from being stepped on or otherwise damaged. While the definition puts the limitation on grain and produce being gathered to prevent damage, gathering for other purposes many violate other melachos instead. Gathereing would include placing produce in containers or stringing them together, as in the case of dates.

The melachah speaks to produce gathered in their place of growth though, gathering in another place may violate other prohibitions. Similarly, while this melachah is directed only towards things that grow from the earth, other prohibitions may be cited in cases of gathering items that do not grow in the ground. Even gathering only two items is a violation of the prohibition.

Gathering is not limited to gathering with ones hands. Pushing the produce or even twigs together with one’s foot is not permissible.

One may take fruit, into their home, one by one so long as the fruit fell from the tree prior to Shabbat. The fruit may not be picked on Shabbat. Also fruit that fell during Shabbat may not be picked up with one’s hands; feet may be used to push the fruit instead.

Gathering for purposes other than preventing damage is not restricted. In the case of cut flowers, gathering for beauty would be permissible. The making of an actual bouquet, however violates another melachah.

While the definition includes items that grow from the earth, the rule has been expanded to apply to thing that come from the earth but do not grow, as in the case of stones or salt.

Additionally, the prohibition addresses fruit, produce or grains gathered in the place that they have grown; the rabbis have extended the application of the rule. Produce that has become scattered inside, in a large area, has the same status as produce scattered in a large outside area.

The rule for a large outside area is that produce, previously gathered, may not be regathered on Shabbat. They may be gathered, for use on Shabbat, if scattered in a small area in a yard. This rule only applies if the collection of the items in the small area would not require a large degree of toil or bother.

Biblically, dates may not be strung together whether indoors or out.

Adapted from the Shabbos Home by Rabbi Simcha Bunim Cohen