She was afraid of the dog at the gate.

Literal

She [topic-は] gate [possessive-の] place [to-に] exist dog [subject-が] scary was.

門のところにいる犬が怖かった — the relative clause (門のところにいる, 'that is at the gate') modifies 犬. 怖い takes が for the scary thing. The past tense indicates she was afraid at that time.