Origins
Together is a tautology, and gather is the proof. Both words descend from Proto-Germanic *gadą, meaning 'together' or 'companionship'. Old English tōgædere literally means 'to-togetherness' — the 'together' meaning was already baked in, and the prefix tō- just reinforced it.
Old English gaderian meant 'to collect, to unite, to bring into one place'. It was a practical, physical word — gathering crops, gathering people, gathering firewood. The Proto-Germanic root *gadurōną carried the same sense of bringing things that belong together into actual togetherness.
The inferential sense — 'I gather you're not coming' — appeared in the 16th century. It grew from the collecting metaphor: to gather information was to collect scattered facts, and drawing a conclusion from them was a natural extension. You gather evidence, then you gather meaning.
Later History
In sewing, a gather is a fold created by pulling fabric together — a small, precise instance of the word's original sense. Gathering storms work similarly: clouds collect, drawing together before releasing.
The word has stayed remarkably close to its root meaning across a thousand years. Where Latin-derived synonyms like collect and assemble carry formal weight, gather remains earthy and direct — an Anglo-Saxon word for an Anglo-Saxon activity: bringing things together with your hands.