。
She was watching the passengers get off the bus.
Literal
She [topic-は] passengers [subject-が] bus [object-を] getting-off [nominalizer-の] [object-を] was-gazing-at.
見つめる (みつめる, 'to gaze at, to stare at') is more intense than 見る ('to look/see') — it implies sustained, focused attention. The ~のを見つめる construction parallels ~のを聞く for perception: she observed the event of passengers disembarking in real time. The polite ていました places this in a narrative context.