One is used in an active sentence, and one is used in a passive sentence.
Put simply, gathering is a verb, while being gathered is a state or condition.
I am gathering wood for a fire.
(Active sentence: the subject is doing the action.
Gathering is the action the subject does.)
Wood is being gathered for a fire.
(Passive sentence: the subject is receiving the action, instead of doing it.
The wood is in the state of being gathered by someone or something else that is not stated.)