。
She gave the door a gentle push.
Literal
She [topic-は] door [object-を] gently pushed.
そっと is a 擬態語 (state mimetic) for doing something gently, quietly, and unobtrusively — the manner of a tip-toer, a worried mother peeking into her child's room, someone trying not to disturb. Distinct from ゆっくり ('slowly') in flavor: そっと is about gentleness and stealth, ゆっくり about pace. Mimetics like this carry a richness of nuance that's almost impossible to translate into a single English word, which is why Japanese uses them so prolifically.