。
She left long before you arrived.
Literal
She [topic-は] you [subject-が] arrive [much-ずっと] before, here [object-を] left.
Two pieces worth noticing. First, the embedded relative clause '君が着く' ('you arrive') uses が rather than は to mark its subject — this is the regular pattern for subjects inside subordinate clauses, where the topic-marker は doesn't easily reach. Second, ずっと前に ('much before, long before') intensifies the temporal gap: ずっと is an adverb expressing 'far, way, much,' applied here to the time-axis. を with 去る ('leave/depart') marks the place being departed from — を can be the marker of the path/source for verbs of motion, not just direct objects.