She came into the garden and stood gazing at the trees for a long time.
Literal
She [topic-は] garden [to-へ] entered-came [and-て], long time trees [object-を] was-gazing.
入ってきて uses ~てくる for motion toward the speaker's location — she came into the garden (where the narrator is). 眺めていました uses the polite ~ていました for a past progressive: 'was gazing.' 眺める ('to gaze / look out at') is more contemplative than 見る — it implies unhurried, appreciative looking. 長い間 ('for a long time') is a common duration expression.